The document discusses various aspects of highway engineering related to horizontal and vertical alignment of roads. It describes extra widening needed on curved sections of roads to accommodate vehicles. It discusses the analysis and formulas to calculate mechanical and psychological widening. It also covers horizontal transition curves, their objectives and methods to determine length. The document discusses setback distance for obstructions on curved sections and the formulas to calculate setback based on sight distance and curve length. It concludes with definitions of gradient, ruling gradient and other types for vertical alignment considerations.

1. Highway Engineering by Rocksana Akter
Extra widening
Extra widening: Extra widening refers to the additional width of carriageway that is required on a curved
section of a road over and above that required on a straight alignment. On horizontal curves, especially when
they are less than 300m radii, it is common to widen the pavement slightly more than the normal width.
Widening is needed for the following reasons:
a. An automobile has a rigid wheel base and only the front wheels can be turned, when this vehicle takes
a turn to negotiate a horizontal curve, the rear wheel do not follow the same path as that of the front
wheels. This phenomenon is called off tracking.
b. While two vehicle cross or overtake at horizontal curve there is psychological tendency to maintain a
greater clearance between the vehicles for safety.
c. For greater visibility at curve, the driver have tendency not to follow the central path of the lane, but
to use the outer side at the beginning of the curve.
d. At higher speed super elevation and lateral friction cannot counteract centrifugal force and skidding
may occur.
Analysis of extra widening on horizontal curves: The extra widening of pavement on horizontal curves is
divided into two parts:
a. Mechanical widening/Off tracking And b. Psychological widening
Mechanical Widening:
The expression for extra width can be
derived from the simple geometry of a
vehicle at a horizontal curve as shown in
figure 4.25. Let R1 is the radius of the
outer track line of the rear wheel, R2 is
the radius of the outer track line of the
front wheel l is the distance between the
front and rear wheel, n is the number of
lanes, then the mechanical widening Wm
is derive below:
OB - OA = R2 – R1 = Wm
From Δ OAB,
OA2
= OB2
– BA2
R2
2
= R1
2
+ l2
= (R2 – Wm) 2
+ l2
= R2
2
– 2R2Wm + W2
m + l2
Or, 2R2Wm – W2
m = l2
Therefore the widening needed for a single lane road is:
Wm =
l2
2𝑅2−𝑊𝑚

2. If the road has n lanes, the extra widening should be provided on each lane. Therefore, the extra widening of
a road with n lanes is given by, Wm =
𝑛l2
2𝑅2−𝑊𝑚
For large radius, R2 ⁓ R, which is the mean radius of the curve, then Wm is given by: Wm =
𝑛l2
2𝑅
Psychological widening:
Widening of pavements has to be done for some psychological reasons also. There is a tendency for the drivers
to drive close to the edges of the pavement on curves. Some extra space is to be provided for more clearance
for the crossing and overtaking operations on curves. IRC proposed an empirical relation for the psychological
widening at horizontal curves Wps:
Wps =
𝑣
2.64√𝑅
Or, Wps =
𝑉
9.5√𝑅
(here, v is mps and V is kmph)
Therefore, the total widening needed at a horizontal curve We is:
We = Wm + Wps
=
𝑛l2
2𝑅
+
𝑣
2.64√𝑅
Q. A National Highway passing through a rolling terrain has two horizontal curves of radius 450 m and
150m. Design the required super elevation for the curves as per IRC guidelines.
Solution:
Assumptions: The ruling design speed for NH passing through a rolling terrain is 80 kmph. The Coefficient
of lateral friction f = 0.15. The maximum permissible super elevation e = 0.07.
Case: Radius = 450m, here, v = V/3.6 = 80/3.6 = 22.22 m/sec
Step 1: For 75 percent of design speed and neglecting f, then e1 =
(0.75𝑣)
2
𝑔𝑅
Or, e1 =
(0.75 𝑥 22.22)
2
9.81 𝑥 450
= 0.0629
Step 2: e1 ≤ 0.07. Hence the design is sufficient.
Answer: Design super elevation is 0.06.
Case: Radius = 150m
Step 1: For 75 percent of design speed and neglecting f, then e1 =
(0.75𝑣)
2
𝑔𝑅
Or, e1 =
(0.75 𝑥 22.22)
2
9.81 𝑥 150
= 0.188 Hence, Max.e to be provided = 0:07
Step 2: Here, f1 =
(0.75𝑣)
2
𝑔𝑅
– e =
(0.75 𝑥 22.22)
2
9.81 𝑥 150
– 0.07 = 0:265.
Step 3: The allowable speed va for the maximum e = 0:07 and f = 0:15,
va = √(0.22𝑔𝑅) = √(0.22 𝑥 9.81 𝑥 150) = 17.99 X 3.6 = 64 kmph
Where, n = no. of traffic lanes
l = length of wheel base =6.1 or 6 m
v,V = design speed mps,kmph
R = radius of the horizontal curve, m

3. Types of Horizontal Curves:
1) Circular Curve and 2) Transition Curve
Horizontal Transition Curves: Transition curve is provided to change the horizontal alignment from straight
to circular curve gradually and has a radius which decreases from infinity at the straight end (tangent point)
to the desired radius of the circular curve at the other end (curve point) There are five objectives for providing
transition curve and are given below:
1. To introduce gradually the centrifugal force between the tangent point and the beginning of the circular
Curve, avoiding sudden jerk on the vehicle. This increases the comfort of passengers.
2. To enable the driver turn the steering gradually for his own comfort and security,
3. To provide gradual introduction of super elevation, and
4. To provide gradual introduction of extra widening.
5. To enhance the aesthetic appearance of the road.
Length of transition curve:
The length of transition curve
is designed to fulfill three
conditions:
a. Rate of change of
centrifugal acceleration to be
developed gradually
b. Rate of introduction of
designed super elevation
c. Minimum length by IRC
empirical formula.
a. Rate of Change of centrifugal acceleration: The length of transition curve is calculated as:
At the tangent point, radius is infinity and hence centrifugal acceleration is zero. At the end of the transition,
the radius R has minimum value R. The rate of change of centrifugal acceleration should be adopted such that
the design should not cause discomfort to the drivers. If C is the rate of change of centrifugal acceleration, it
can be written as:
C =
𝑣2
𝑅
−0
𝑡
=
𝑣2
𝑅
−0
𝐿𝑠
𝑣
=
𝑣3
𝑅 𝐿𝑠
Therefore, the length of the transition curve Ls1 in m is - Ls1 =
𝑣3
𝐶𝑅
or, Ls1 =
0.0215𝑉3
𝐶𝑅
(V is Kmph)
Where C is the rate of change of centrifugal acceleration given by an empirical formula suggested by IRC as
below: C =
80
75+3.6𝑣
(m/sec3
) [0.5 < C <0.8]
Where, Ls = length of transition curve, m
C = allowable rate of change of centrifugal acceleration
v,V = design speed in m/sec,kmph
R = radius of the circular curve, m

4. b. Rate of introduction of super elevation:
(i) If the pavement is rotated about the center line: Ls2 = EN/2 = e N/2(W + We)
(ii) If the pavement is rotated about the inner edge: Ls2 = EN = e N(W + We)
Where, W = width of pavement, We = extra widening, E=total raised pavement = e. B, B=total width of
pavement = (W + We), Rate of change of super elevation of 1 in N, N=150, 100, 60.
c. By empirical formula: IRC suggest the length of the transition curve is minimum for a plain and
rolling terrain: Ls3 =
35𝑣2
𝑅
and for steep and hilly terrain is: Ls3 =
12.96𝑣2
𝑅
And the shift s as: s =
𝐿𝑠
2
24𝑅
Setback Distance
Setback Distance: Setback distance m or the clearance distance is the
distance required from the centerline of a horizontal curve to an obstruction
on the inner side of the curve to provide adequate sight distance at a
horizontal curve. The setback distance depends on:
1. Sight distance (OSD, ISD and OSD),
2. Radius of the curve, and
3. Length of the curve.
Analysis of Setback distance: Two cases are considered for the analysis:
a. When length of curve is more than Sight distance (Lc > S)
b. When length of curve is less than Sight distance (Lc < S)
Case (a): When Lc >S:
For single lane roads: α =
𝑠
𝑅
radians =
180𝑠
𝜋𝑅
degrees
Or, α/2 =
180𝑠
2𝜋𝑅
degrees
Therefore, m = R – Rcos(α/2)
For multi lane roads: If d is the distance between centerline of the
road and the centerline of the inner lane, then,
m = R – (R – d) cos (
180𝑠
2𝜋(𝑅−𝑑)
)
Where, α =angle subtended by the arc
m = set back distance, m and, R=radius of curve, m
s = Sight Distance, m
d = distance between center line of road and center line of inner
lane.

5. Case (b): When Lc < S:
For single lane roads: m1 = R – Rcos(α/2)
m2 =
(𝑆 − 𝐿𝑐)
2
sin (α/2)
The set back is the sum of m1 and m2 given by:
m = R – Rcos(α/2) +
(𝑆 − 𝐿𝑐)
2
sin(α/2). Where, α/2 =
180𝐿𝑐
2𝜋𝑅
For multi-lane road: α/2 =
180𝐿𝑐
2𝜋(𝑅−𝑑)
and m is given by ---
m = R – (R – d) cos (α/2) +
(𝑆 − 𝐿𝑐)
2
sin (α/2)
Q: Find out the available setback from an obstruction for 4 degree horizontal curve where sight distance
required is 350 m (550 m). The length of the curve is 600 m (400 m).
Solution:
Here, S = 350m and L = 600m, Hence, S < L
θ = 4o;
Now,
2𝜋𝑅
360
=
𝑆
θ
or,
2𝜋𝑅
360
=
350
4
or, R = 5013.38m
So, m =
𝑆2
8𝑅
=
3502
8 𝑥 5013.38
= 3.05m
Again, S = 550m and L = 400m, Hence, S > L; Now,
2𝜋𝑅
360
=
𝑆
θ
Or,
2𝜋𝑅
360
=
400
4
Or, R = 5729.57m
So, m =
𝐿𝑥(2𝑆−𝐿)
8𝑅
=
400𝑥(2𝑥550−400)
8𝑥5729.57
= 6.11m
Vertical Alignment
Vertical Alignment: The vertical alignment is the elevation or profile of the center line of the road. The
vertical alignment consists of grades and vertical curves.
The vertical alignment of a highway influences:
i. Vehicle speed
ii. Acceleration and deceleration
iii. Sight distance
iv. Vehicle operation cost
v. Comfort while travelling at high speeds
Gradients: Gradient is the rate of rise or fall along the length of road with respect to the horizontal. It is
expressed as a ratio of 1 in n or also as percentage such as n%. Gradients are divided into four categories:
a. Ruling gradient, b. Limiting gradient, c. Exceptional gradient, d. Minimum gradient.
Ruling gradient: The ruling gradient or the design gradient is the maximum gradient with which the designer
attempts to design the vertical profile of the road. This depends on the terrain, length of the grade, speed,
pulling power of the vehicle and the presence of the horizontal curve.
In flatter terrain, it may be possible to provide at gradients, but in hilly terrain it is not economical and
sometimes not possible also. The ruling gradient is adopted by the designer by considering a particular speed
as the design speed and for a design vehicle with standard dimensions. But our country has a heterogeneous
Here, S is the curve portion, which is the
lower value between given S and L.

6. traffic and hence it is not possible to lay down precise standards for the country as a whole. Hence IRC has
recommended some values for ruling gradient for different types of terrain.
Limiting gradient: This gradient is adopted when the ruling gradient results in enormous increase in cost of
construction. On rolling terrain and hilly terrain it may be frequently necessary to adopt limiting gradient. But
the length of the limiting gradient stretches should be limited and must be sandwiched by either straight roads
or easier grades. It depends on topography and cost in constructing the road.
Exceptional gradient: Exceptional gradient are very steeper gradients given at unavoidable situations. They
should be limited for short stretches not exceeding about 100 meters at a stretch. In mountainous and steep
terrain, successive exceptional gradients must be separated by a minimum 100 meters length gentler gradient.
At hairpin bends, the gradient is restricted to 2.5%.
Type of terrain Ruling gradient Limiting gradient Exceptional gradient
Plain and rolling 3.3 %
1 in 30
5%
1 in20
6.7 %
1 in 15
Mountainous and steep having elevation
more than 3000 m above MSL
5%
1 in20
6 %
1 in 16.7
7 %
1 in 14.3
Mountainous and steep having elevation
more than 3000 m above MSL
6 %
1 in 16.7
7 %
1 in 14.3
8 %
1 in 12.5
Minimum gradient: This is important only at locations where surface drainage is important. Camber will
take care of the lateral drainage. But the longitudinal drainage along the side drains require some slope for
smooth ow of water. Therefore minimum gradient is provided for drainage purpose and it depends on the rain
fall, type of soil and other site conditions. A minimum of 1 in 500 may be sufficient for concrete drain and 1
in 200 for open soil drains are found to give satisfactory performance.
Grade Compensation: Grade compensation can be defined as the reduction in gradient at the horizontal curve
because of the additional tractive force required due to curve resistance, which is intended to offset the extra
tractive force involved at the curve. Grade compensation, % =
30+𝑅
𝑅
.
IRC gave the following specification for the grade compensation:
1. Grade compensation is not required for grades flatter than 4% because the loss of tractive force is negligible.
2. Grade compensation is
30+𝑅
𝑅
%, where R is the radius of the horizontal curve in meters.
3. The maximum grade compensation is limited to 75 R %.
Compensated gradient = Ruling gradient – Grade compensation
Vertical Curve: The vertical curve is a parabolic curve that is applied to make
a smooth and safe transition between two grades on a roadway or a highway.
Here, in figure; VPC = Vertical Point of Curvature, VPI = Vertical Point of
Intersection, VPT = Vertical Point of Tangency,
G1 and G2 = Tangent grades in percent, A = Algebraic difference in grades,
L = Length of vertical curve
Q. When are vertical curve applied?
Ans: The vertical curve is applied at an intersection of two slopes on a highway or a roadway and to provide
a safe and comfort ride for vehicles on a roadway.
K – Value: The horizontal distance in feet (meters) needed to make 1% change in gradient. I.e. K =
𝐿
𝐴

7. Application of K - Value:
 To determine the minimum lengths of vertical curves.
 To determine the horizontal distance from the VPC to the high point of Type I or the low point of Type
III.
Crest vertical Curve: Minimum length of a crest vertical curve needs to satisfy the safety, comfort, and
appearance criteria. Minimum length of a crest vertical curve is equal 3 time the design speed (only for English
Unit).
General equation for the length
of a crest vertical curve in terms
of algebraic difference in
grades.
a. When S is less than L
Then, L =
𝐴𝑆2
100(√2ℎ1+√2ℎ2)
2
b. When S is greater than L
Then, L = 2S -
200(√ℎ1+√ℎ2)
2
𝐴
Where, L = length of vertical curve in ft, S = sight distance in ft, A = algebraic difference in grades in percent
h1 = height of eye above roadway in ft (3.5ft), h2 = height of object above roadway surface in ft (2ft)
Sag vertical curve: A design of a sag vertical curves need to satisfy at least four difference criteria. Such as:
General equation of sag vertical curve:
When S < L, Then -
L =
𝐴𝑆2
200(ℎ1+𝑆 𝑡𝑎𝑛𝛽)
When S > L, Then –
L = 2S -
200(ℎ1+𝑆 𝑡𝑎𝑛𝛽)
𝐴
Where, L= length of sag vertical cure in ft, S= light beam distance in ft, A = algebraic
difference in grades in percent, 𝛽 = angle of light beam intersects the surface of the roadway,
degree (assumed 1o), h1 = head light height, (assumed 2ft)
For passenger comfort, the below equation can be used. L =
𝐴𝑉2
46.5
Here, L= length of sag vertical curve in ft, A = algebraic difference in grades in percent, V =
design speed in mph.
K values are calculated by equation: K =
𝑆2
400+3.5𝑆

8. Summit Curve: Summit curves are vertical curves with gradient upwards. Design of summit curve as a square
parabola because the rate of change of slope is decreasing always so more sight distance available at the top
of curve. They are formed when two gradients meet as illustrated in figure in any of the following four ways:
1. When a positive gradient meets another positive gradient [Figure 17:2a].
2. When positive gradient meets at a gradient [Figure 17:2b]. .
3. When an ascending gradient meets a descending gradient [Figure 17:2c]. .
4. When a descending gradient meets another descending gradient [Figure 17:2d].
Length of the summit curve: Equation of the parabola is given by y = ax2, where a =
𝑁
2𝐿
, where N is the
deviation angle and L is the length of the In deriving the length of the curve, two situations can arise depending
on the uphill and downhill gradients when the length of the curve is greater than the sight distance and the
length of the curve is greater than the sight distance.
Let L is the length of the summit curve, S is the SSD/ISD/OSD, N is the deviation angle, h1 driver's eye height
(1.2 m), and h2 the height of the obstruction, then the length of the summit curve can be derived for the
following two cases. The length of the summit curve can be derived from the simple geometry as shown
below:
Case –I: Length of summit curve is greater than sight distance (L>S)
y = ax2
here, a =
𝑁
2𝐿
H = aS1
2
and h2 = aS2
2
Now, S1 = √ (
𝐻
𝑎
) and S2 = √ (
ℎ2
𝑎
)
So, S1 + S2 = √ (
𝐻
𝑎
) + √ (
ℎ2
𝑎
)
Or, S2
= (
1
√𝑎
)2
(√𝐻+√ℎ2)2
Or, S2
=
2𝐿
𝑁
(√𝐻+√ℎ2)2
Therefore, L =
𝑁𝑆2
2(√𝐻+√ℎ2)2
Case II: Length of summit curve less than sight distance (L<S)
From the basic geometry, one can write - S =
𝐿
2
+
ℎ1
𝑛1
+
ℎ2
𝑛2
=
𝐿
2
+
ℎ1
𝑛1
+
ℎ2
𝑁− 𝑛1
Therefore for a given L, h1 and h2 to get minimum S, differentiate the above equation with respect to h1 and
equate it to zero. Therefore,
𝑑𝑆
𝑑ℎ1
= -
ℎ1
𝑛1
2 +
ℎ2
𝑁 − 𝑛1
2 = 0

9. Solving for n1,
Now we can substitute n back to get the
value of minimum value of L for a given
n1, n2, h1 and h2.
Therefore,
Solving for L,
When stopping sight distance is considered the height of driver's eye above the road surface (h1) is taken as
1.2 meters, and height of object above the pavement surface (h2) is taken as 0.15 meters. If overtaking sight
distance is considered, then the value of driver's eye height (h1) and the height of the obstruction (h2) are taken
equal as 1.2 meters.
Valley Curve: Valley curve or sag curves are vertical curves with convexity downwards. In day time, no
problem of any sight distance but in night time head light sight distance should be sufficient for vehicle to
stop before colliding with object. In valley curves, the centrifugal force will be acting downwards along with
the weight of the vehicle. This will result in jerking of the vehicle and cause discomfort to the passengers.
They are formed when two gradients meet as illustrated in figure 18:1 in any of the following four ways:
1. When a descending gradient meets another descending gradient [Figure 18:1a].
2. When a descending gradient meets at a gradient [Figure 18:1b].
3. When a descending gradient meets an ascending gradient [Figure 18:1c].
4. When an ascending gradient meets another ascending gradient [Figure 18:1d].
Length of the valley curve: The valley curve is made fully transitional by providing two similar transition
curves of equal length The transitional curve is set out by a cubic parabola y = bx3
where, b =
2𝑁
3𝐿2
The length
of the valley transition curve is designed based on two criteria:

10. 1. Comfort criteria: That is allowable rate of change of centrifugal acceleration is limited to a comfortable
level of about 0.6m/sec3.
2. Safety criteria: That is the driver should have adequate headlight sight distance at any part of the country.
Comfort criteria: The length of the valley curve based on the rate of change of centrifugal acceleration that
will ensure comfort:
Let c is the rate of change of acceleration, R the minimum radius of the curve, v is the design speed and t is
the time, then c is given as:
C =
𝑣2
𝑅
−0
𝑡
=
𝑣2
𝑅
−0
𝐿𝑠
𝑣
=
𝑣3
𝑅 𝐿𝑠
therefore, Ls =
𝑣3
𝐶𝑅
For a cubic parabola, the value of R for length Ls is given by: R =
𝐿𝑠
𝑁
Therefore,
Where, c =
80
75+3.6𝑣
here, v is in m/sec,
L is the total length of valley curve,
N is the deviation angle in radians or tangent of the deviation angle or
the algebraic difference in grades, and
c is the allowable rate of change of centrifugal acceleration which may
be taken as 0.6m/sec3
.
Safety criteria: Length of the valley curve for headlight distance may be determined for two conditions: (1)
length of the valley curve greater than stopping sight distance and (2) length of the valley curve less than the
stopping sight distance.
Case-I: Length of valley curve greater than stopping sight distance (L > S)
The total length of valley curve L is greater than the stopping sight distance SSD. The sight distance available
will be minimum when the vehicle is in the lowest point in the valley. This is because the beginning of the
curve will have infinite radius and the bottom of the curve will have minimum radius which is a property of
the transition curve. The case is shown in figure 18:2 from the geometry of the figure, we have –
h1 + S tanα = aS2
=
𝑁𝑆2
2𝐿
So, L =
𝑁𝑆2
2(ℎ1 + S tanα )
Where,
N = the deviation angle in radians,
h1 = the height of headlight beam,
α = the head beam inclination in
degrees and
S = the sight distance.
The inclination α is ~1 degree.

11. Case-II: Length of valley curve less than stopping sight distance (L < S)
The length of the curve L is less than SSD. In this case the minimum sight distance is from the beginning of
the curve. The important points are the beginning of the curve and the bottom most part of the curve. If the
vehicle is at the bottom of the curve, then its headlight beam will reach far beyond the endpoint of the curve
whereas, if the vehicle is at the beginning of the curve, then the headlight beam will hit just outside the curve.
Therefore, the length of the curve is derived by assuming the vehicle at the beginning of the curve. The case
is shown in figure 18:3.
From the figure,
h1 + s tanα = (S −
𝐿
2
) N
L = 2S −
2ℎ1+2𝑆 𝑡𝑎𝑛𝛼)
𝑁
Note that the above expression is approximate
and is satisfactory because in practice, the
gradients are very small and is acceptable for
all practical purposes. We will not be able to
know prior to which case to be adopted.
Therefore both has to be calculated and the one
which satisfies the condition is adopted.
Roadway
Highways: They represent the superior type of roads in the country. Highways are of two types – rural
highways and urban highways. Rural highways are those passing through rural areas (villages) and urban
highways are those passing through large cities and towns, i.e. urban areas.
A. Geographical area or location wise:
(i) Rural Road and (ii) Urban Road
B. Function Wise:
Rural Road: National Highways, Regional Highways, Feeder Road A and Feeder Road B.
Urban Road: Primary/Main/Arterial Road, Secondary Road and Local Road.
C. Standard wise:
a. Full access control,
Expressway/Motorway
with grade separated
junctions
b. Partial/No access
control with at-grade
intersection.
Freeways: Freeways are access-controlled divided highways. Most freeways are four lanes, two lanes each
direction, but many freeways widen to incorporate more lanes as they enter urban areas. Access is controlled
through the use of interchanges, and the type of interchange depends upon the kind of intersecting road way
(rural roads, another freeway etc.)
Expressways: They are superior type of highways and are designed for high speeds (120 km/hr is common),
high traffic volume and safety. They are generally provided with grade separations at intersections. Parking,
loading and unloading of goods and pedestrian traffic is not allowed on expressways.

12. Arterials: It is a general term denoting a street primarily meant for through traffic usually on a continuous
route. They are generally divided highways with fully or partially controlled access. Parking, loading and
unloading activities are usually restricted and regulated. Pedestrians are allowed to cross only at
intersections/designated pedestrian crossings.
Collector streets: These are streets intended for collecting and distributing traffic to and from local streets
and also for providing access to arterial streets. Normally full access is provided on these streets. There are
few parking restrictions except during peak hours.
Local streets: A local street is the one which is primarily intended for access to residence, business or abutting
property. It does not normally carry large volume of traffic and also it allows unrestricted parking and
pedestrian movements.
D. Usage wise
a) Commuter road
b) All-purpose road
c) By-pass
d) Distributor/Ring road
e) Feeder/Collector road
f) Slip/Access road
g) Service road
h) Frontage road
Road Intersection
Road Intersection: When two or more roads intersect each other, it is called Intersections or Road Junction.
Road intersections should be designed to provide safety to road users and free flow of traffic. This area is
designated for the vehicles to turn to different directions to reach their desired destinations. This is because
vehicles moving in different direction want to occupy same space at the same time.
The factors to be considered while making selection for a particular type of road junction are as follows:
ity of means of traffic control
Following are the general principles to be observed in the design of road junctions to minimize the risks
of accidents:
•Angle of crossing, •Camber, •Channelizing, •Entry speeds, •Gradient, •Importance of roads, •Kerbs,
•Pedestrian, •Simplicity, •Visibility.
Road Intersection are two types. Such as:
(i)At grade intersections (Un-channelised, Rotary and Channelised Rotary), (ii) Grade separated intersection
At Grade Intersections: An intersection is a junction where two or more roads meet or cross. Roads that
cross each other at the same level or elevation are called At grade Intersections.
1. Un-Channelised At Grade Intersections: These are At-grade intersections without islands for
directing traffic into definite paths.These are simple in design and dangerous in traffic operations.
These are classified into :-
a.Square junction
b.Skew junction or Acute junction
c.T- junction
E. Operation wise
a) Single Carriageway/Undivided
road
b) Dual Carriageway/ Divided
road
c) Two way
d) One way
e) Lane/non-lane based
F. Investment wise
a) Public road- free
b) Private: BOT-
tolled road

13. d.Y- junction
e.Multiple junction
2. Channelised At Grade Intersections: At these intersections islands are introduced to reduce the total
conflict area available. These islands are useful to channelise the turning traffic, to control the speed and angle
of approach. Vehicles approaching an intersection are directed to definite paths by islands, marking etc. and
this method of control is called channelization.
•It provides more safety and efficiency.
•It reduces the number of possible conflicts by reducing the area of conflicts available in the carriageway.
•The presence of traffic islands, markings etc. forces the driver to reduce the speed and becomes more cautious
while maneuvering the intersection.
•It also serves as a refuge for pedestrians and makes pedestrian crossing safer.
3. Rotary Intersection: The rotary intersection of roads is also called as traffic rotary which is nothing but
enlarged intersection of roads where vehicles cross roads or change their direction without stopping. All
vehicles coming from different roads move in single direction around the central island and diverges into
required exit.
Grade Separated Intersection: This are the intersections at which the vehicles crosses at different levels.
They cause less hazard and delay than grade intersections. The objective of this is to eliminate all grade
crossing conflicts and to accommodate other intersecting maneuvers by merging, diverging and weaving at
low relative speed. It increases the capacity of roads and reduce the congestions.
Underpass: It is an underground passageway, completely enclosed except at the openings, commonly at each
end. A tunnel may be for foot or vehicular road traffic, for rail traffic. If an underpass is constructed for
pedestrians and/or cyclists beneath a road or railway, allowing them to reach the other side in safety, then such
a construction is termed as a Subway.
Overpass: It is also known as a flyover, is a bridge, road, railway or similar structure that crosses over another
road or railway. A pedestrian overpass allows pedestrians safe crossing over busy roads without impacting
traffic. And Railway overpasses are used to replace at- grade crossing as a safer alternative. Overpasses allows
for unobstructed rail traffic flow from mixing with vehicular and pedestrian traffic.
Trumpet Interchange: It is a three leg interchange. If one of the legs of the interchange meets a highway at
some angle but does not cross it, then the interchange is called Trumpet Interchange. It is used where one
highway terminates at another highway. They are trumpet shaped intersection. These involve at least one loop
ramp connecting traffic either entering or leaving the terminating expressway with the far lanes of the
continuous highway. Trumpets are suitable at the locations where the side road exists on only one side of the
freeway, and traffic is relatively low. The principal advantages are low construction cost and are useful for
highways as well as toll roads.
Diamond Interchange: It is the simplest form of grade separated intersection between two roadways. The
conflicts between through and crossing traffic are eliminated by a bridge structure. The left turn crossing
movement conflicts are considerably reduced by eliminating the conflict with the traffic in opposite direction.
It is suitable for locations where the volume of left turn traffic is relatively low. It requires a minimum amount
of land and is economical to construct. The single point of exit from the major roadway eases the problem of
signing.
Cloverleaf Interchange: A cloverleaf interchange is a two-level interchange in which left turns are handled
by ramp roads .To go left vehicles first continue as one road passes over or under the other, then exit right
onto a one-way three-fourths loop ramp (270°) and merge onto the intersecting road. The objective of a
cloverleaf is to allow two highways to cross without the need for any traffic to be stopped by red lights, even
for left and right turns. The limiting factor in the capacity of a cloverleaf interchange is traffic weaving.
Partial Cloverleaf Interchange: It is a modification that combines some elements of a diamond interchange
with one or more loops of a cloverleaf to eliminate only the more critical turning conflicts. This is the most
popular freeway -to- arterial interchange. It is usually employed when crossing roads on the secondary road
will not produce objectionable amounts of hazard and delay. It provides more acceleration and deceleration
space on the freeway.
Directional Interchange: A Directional interchange provides direct paths for left turns. These interchanges
contain ramps for one or more direct or semi direct left turning movements. They are the basic patterns that
use the least space, have the fewest or least complex structures, minimize internal weaving and appropriate
for the common terrain and traffic conditions.

14. Limitations:
•High cost of construction
•It requires large amount of land compared to the others.
Traffic Islands
Traffic Island: When two or more roads of equal importance intersect at a point traffic islands were
introduced and all vehicles are compelled to move along the same direction only.
There are different types of traffic islands. They are:-
1. Circular Islands
2. Turbine Island
3. Rhombus Islands
4. Tangent Island
Design elements:
1. Design Speed
•It should be passed with low speed which will results the safety without any collision.
•Speed restriction board should be provided on the road towards rotary intersection.
2. Shape of Central Island
•The shape of Central Island should not contain any corners.
•It should be formed by curves to allow the comfortable rotations around it.
•The shape is dependent upon number of roads meeting at that junction.
•The shapes generally provided are circular, elliptical, turbine and tangential.
3. Radius of Rotary Roadway
•The radius of roadway is dependent upon the shape of Central Island and the frictional force.
•If it is circular shape, radii are similar at all points and if it is elliptical or tangent radii is different at
different points.
4. Weaving Angle and Weaving Distance
•The weaving angle should be small but minimum of 15ois maintained.
•The weave length should be at least four times the width of weaving section.
5. Width of Carriageway at Entry and Exit
•The width of carriage way at entry and exits is dependent of volume of traffic in that particular region or
area.
•Minimum width of 5.0 meters should be maintained for rotary intersections.
6. Width of Rotary Roadway
•The width should be equal to the effective width of weaving section.
7. Curves at Entrance and Exit
•Entry curve radius can be provided as same as radius of central island.
•The vehicle accelerates at exits hence the radius of curve at exit should be greater than the radius of curve
at entrance.

15. 8. Sight Distance
•The sight distance provided at rotary intersections should be as higher as possible and value must be less
than the stopping sight distance.
9. Camber and Super Elevation
•If the vehicle is changing its direction to its opposite side, the vehicle may over turn or slip, to overcome
this, minimum cross slope is provided which is nothing but camber.
•This camber acts as super elevation in case of rotary roadways.
Traffic Engineering
Traffic Engineering: Traffic engineering is a branch of civil engineering that uses engineering techniques to
achieve the safe and efficient movement of people and goods on roadways.
Traffic Engineering Includes:
• The study of regulations for traffic
• Characteristics of traffic
• Controlling and guiding measures for traffic
• Flow of traffic at junctions
• Parking areas
• Traffic survey
• Engineering elements of road system affecting traffic operation
• Improvement of traffic facilities in existing towns and so on.
Objects of Traffic Engineering:
• To achieve smooth and easy flow of traffic at intersections
• To develop methods for improvement in general and for solving specific problems in particular
• To have safe, convenient, rapid and economic transport of persons and goods
• To improve the speeds of vehicles
• To increase the traffic carrying capacity of roads
• To make the streets safe for the movement of both, pedestrians and vehicles
• To reduce delays in road journeys
• To reduce the chances of road accidents to a minimum
• To remove traffic congestion
Traffic Survey or Study:
Aims of traffic survey:
• To analyze the road accidents and to find out road elements contributing to their occurrence
• To determine the facilities provided on road and to suggest the measures to improve the
• To get data for suitable geometric design of various components of roads
• To obtain the knowledge of the nature of traffic at present and to forecast its future trend
• To provide suitable parking facilities
• To suggest controlling measures for speed
Types of Traffic Survey:
• Accident survey
• Origin and destination survey
• Parking survey
• Spot speed survey
• Speed and delay survey
• Traffic volume survey
Accident Survey: The investigations to be carried out during an accident survey include the following items:
•Details of vehicles such as registration number, type of vehicle, loading details.
•Estimation of cost of accident
•General data such as time, date, and person involved, etc.
•Location of the accident
•Nature of accident including injuries, damages etc.
•Primary causes of accident
•Road and traffic conditions at the site of accident.

16. Road Accident
Road Accident:
Causes of Road accident:
a) Defective vehicles
b) Drivers
c) Moving of animals
d) Passengers
e) Pedestrians
f) Road design
g) Traffic volume
h) Weather
The problems of traffic on road result from the performance and requirements of the following three
components:
(i) Fixed facilities for the accommodation of traffic on the road
(ii) Human beings using the road
(iii) Vehicles on the road
All the traffic controls and improvements can be reduced to the behavior of the above three components.
Three E: It has been slightly said that the traffic control methods can be expressed by three E’s.
i.e Education, Enforcement and Engineering.
Education: Educational methods include training of public opinion for respecting traffic rules and regulations
by means of lectures, films, posters, radio talks, articles in newspapers, safety drive programmers, etc.
Enforcement: Enforcement methods include framing of a proper traffic code, appointing an authority to
supervise the road traffic and punishing the defaulters of traffic code, introducing proper signal system,
medical checkup of drivers, special precautions for commercial vehicles, speed control, etc.
Engineering: Engineering methods include suitable street systems, proper design of junctions, land use
zoning, location of public bus stops, physical features of road, parking facilities, proper road lighting etc.
For the safety of pedestrians on road, the following measures may be taken:
1) The fencing and railing is provided
2) The pedestrians are completely segregated at road junctions and they are not allowed to enter the road
intersections.
3) The traffic islands with proper markings
4) The shopping entrances on roads are provided with telescopic railing
5) The vehicles are allowed to move in the urban areas at certain restricted speeds.
Origin and Destination Survey: The details collected during O & D survey help in taking decisions with
respect to the following items:
1) Establishing preferential routes for various categories of vehicles
2) The location of new bridge or tunnels as per demand and intensity of traffic
3) The location of new proposed roads
4) The location of parking places
5) The necessity of making one way street
6) The necessity and usefulness of new roads
7) To regulate movement of heavy vehicles
8) To stop the movement of slow moving vehicles during certain hours of day
Most commonly adopted methods for O & D survey:
1) House interview method
2) License plate method
3) Postal questionnaire method
4) Route interview method
5) Tag on car method
Home work: Details, Advantages and Disadvantages

17. Parking Study
Parking: Parking studies mainly for car. Usually conducted for urban areas with demand for parking. Parking
is essential for business and commercial activities. It should be an integral part of roadway system and
provision of off-street parking spaces should be considered at the urban planning stage. It is roughly estimated
that out of 8760 hours in a year, the car runs on an average for only 400 hours (4.5%) leaving 8360 hours
when it is parked. Parking control has potential to improve roadway capacity and safety.
Objectives of Parking Studies:
•To obtain information regarding supply and demand of parking spaces
•To develop systematic on-street parking facilities/regulations
•To improve existing on street facilities
•To develop new off-street parking facilities
Parking Studies Included:
1) Supply Surveys- To know parking supply/space available in the area under consideration by
conducting –
 Space Inventories- listing all the existing parking facilities and open spaces available for
adopting as parking place as well as listing areas where parking should be prohibited (i.e. Bus
stops, clear-ways, drive-ways/entrances etc.)
2) Demand Surveys- To know parking demand including even who are searching for spaces by
conducting –
 Field Observation- recording location, type of vehicle, number of parking vehicles, duration of
parking etc.
 Interview- Questionnaire survey
Parking should be prohibited at:
•Intersections, •Narrow streets, •Driveways, •Pedestrian crossing, •Curvature and grade conditions
•Road bridges and tunnel, •Heavy pedestrian concentration areas
•Priority location-bus stops, fire hydrants etc.
Some of the terms used in the parking survey as follows:
1) Parking accumulation: It is defined as the number of vehicles parked at a given instant of time. Normally
this is expressed by accumulation curve. Accumulation curve is the graph obtained by plotting the number of
bays occupied with respect to time. During peak hours, the parking accumulation is from 70 percent to 90
percent of the traffic accumulated.
2) Parking Duration: The length of time for which a vehicle remains in the parked position is known as the
parking duration.
3) Parking load: Parking load gives the area under the accumulation curve. It can also be obtained by simply
multiplying the number of vehicles with the time interval. It is expressed as vehicle hours.
4) Parking turnover: The ratio of the number of vehicles parked during a period to the total parking capacity
is known as parking turnover. If the parking turnover is high, it indicates well utilization of parking space.
5) Parking volume: Parking volume is the total number of vehicles parked at a given duration of time.
6) Parking index: Parking index is also called occupancy or efficiency. It is defined as the ratio of number of
bays occupied in a time duration to the total space available. It gives an aggregate measure of how effectively
the parking space is utilized. Parking index can be found out as follows:
Therefore, Parking Index =
𝑃𝑎𝑟𝑘𝑖𝑛𝑔 𝐿𝑜𝑎𝑑
𝑃𝑎𝑟𝑘𝑖𝑛𝑔 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦
x 100
The parking facilities can be grouped in the following two categories:
1) Kerb or On-street parking And, 2) Off-street parking
Problems associated with uncontrolled on-street parking:
•Reduce roadway capacity
•Causes congestion
•Causes serious economic losses to the community
•Encourage road side non motor activities (increase side frictions) •Unmarked/uncontrolled parking leads less
efficient use of road side spaces
•Loss of revenue
•Makes roadway operation unsafe and haphazard

18. On-street parking: On street parking means the vehicles are parked on the sides of the street itself. This will
be usually controlled by government agencies itself. Common types of on-street parking are as listed below.
This classification is based on the angle in which the vehicles are parked with respect to the road alignment.
As per IRC the standard dimensions of a car is taken as 5 x 2.5 and that for a truck is 3.75 x 7.5 .
•Vehicles parked along the curb
•Usually for short stay
•Very convenient for the people (less walking)
•Preferably parallel method of parking
•May be of charged or free
Types of on-street parking:
Parallel parking: The vehicles are parked along the length of the road. Here there is no backward movement
involved while parking or unparking the vehicle. Hence it is the safest parking from the accident perspective.
But it consumes the maximum curb length and therefore only a minimum number of vehicles can be parked
for a given kerb length. Since it consumes least width of the road it produces least obstruction to the on-going
traffic on the road. Parallel parking of cars is shown in figure 38:1. The number of vehicles that can be parked,
N =
𝐿
5.9
.
30o Parking: Here more vehicles can be parked compared to parallel parking. Also there is better
maneuverability. It causes minimum delay to traffic. An example is shown in figure 38:2. From the figure, it
is clear that the number of vehicles that can be parked, N =
𝐿−1.25
5
.
45o Parking: As the angle of parking increases, more number of vehicles can be parked. Hence compared to
parallel and thirty degree parking, more number of vehicles can accommodated in this type of parking. From
figure 38:3, number of vehicles that can be parked, N =
𝐿−1.77
3.54
.
60o Parking: Here also more number of vehicles can be accommodated. From the figure 38:4, number of
vehicles that can be parked, N =
𝐿−2.16
2.89
.
Right angle parking: Here the vehicles are parked perpendicular to the direction of the road. Hence it
consumes maximum width. Curb length required is very little. Hence there are chances of severe accidents.
Also it causes obstruction to the road traffic. But it can accommodate maximum number of vehicles.
Off-street parking: There will be some area exclusively allotted for parking which will be at some distance
away from the main stream of traffic. Such a parking is referred to as o_-street parking. They may be operated
by either public agencies or private firms.
•When parking places are provided away from the road curb
•Usually for long stay
•Requires walking of quite a large distance to reach the destination
•Types (back street, surface car parks, multi-storied car parks, basement car parks)
Common causes of parking related accidents:
•Searching parking space and stopping suddenly
•Moving out from stopped position and merging with main stream traffic
•Due to reversing from Parallel Park (hitting children at the rear end)
•Careless opening of the doors of parked vehicles
•Pedestrian crossings near the parked vehicles, etc. (obstruct sight distance)
HW: Methods of Parking.

19. Spot Speed
Spot Speed: Spot speed is the instantaneous speed of a vehicle at a specified location. Spot speed can be used
to design the geometry of road like horizontal and vertical curves, super elevation etc. Spot speed can be
measured using an endoscope, pressure contact tubes or direct timing procedure or radar speedometer or by
time-lapse photographic methods. It can be determined by speeds extracted from video images by recording
the distance travelling by all vehicles between a particular pair of frames.
Factors effect spot speed studies:
•Driver -Age, Gender, motive of the journey, distance of his trip;
•Vehicle -type, age, weight, manufacturer and horse power;
•Roadways and environment -the graphical locations, grade, sight distance, no. of lanes, spacing of
intersections; including time of day and weather
•Traffic –heavy or less volume, density, passing movements, speed regulations;
Data Presentation & Analysis:
See it in this figure:

20. Spot speed data have a number of safety applications, including the following (Robertson 1994):
1. Determining existing traffic operations and evaluation of traffic control devices
a. Evaluating and determining proper speed limits
b. Determining the 50th and 85th speed percentiles (explained below)
c. Evaluating and determining proper advisory speeds
d. Establishing the limits of no-passing zones
e. Determining the proper placements of traffic control signs and markings
f. Setting appropriate traffic signal timing
2. Establishing roadway design elements
a. Evaluating and determining proper intersection sight distance (for more information refer to Chapter
4 in this handbook)
b. Evaluating and determining proper passing sight distance (for more information refer to
Chapter 3 in the AASHTO Green Book)
c. Evaluating and determining proper stopping sight distance (for more information refer to
Chapter 4 in this handbook)
3. Assessing roadway safety questions
a. Evaluating and verifying speeding problems
b. Assessing speed as a contributor to vehicle crashes
c. Investigating input from the public or other officials
4. Monitoring traffic speed trends by systematic ongoing speed studies
5. Measuring effectiveness of traffic control devices or traffic programs, including signs and markings,
traffic operational changes, and speed enforcement programs
Spot speed data are gathered using one of three methods:
(1) Stopwatch method, (2) radar meter method, or (3) pneumatic road tube method.
The 50th percentile is the median speed of the observed data set. This percentile represents the speed at which
half of the observed vehicles are below and half of the observed vehicles are above. The 50th
percentile of
speed represents the average speed of the traffic stream.
The 85th percentile is the speed at which 85% of the observed vehicles are traveling at or below. This
percentile is used in evaluating/recommending posted speed limits based on the assumption that 85% of the
drivers are traveling at a speed they perceive to be safe (Homburger et al. 1996). In other words, the 85th
percentile of speed is normally assumed to be the highest safe speed for a roadway section.
Solution of 2.1:
The 50th and 85th speed percentiles are determined
from the cumulative percent column. For the
example data in Table 2.1, the 50th percentile falls
between 27 and 30 mph and the 85th
percentile falls
between 33 and 36 mph. The calculation of speed
percentiles is easier if a sample size of 100 vehicles
is collected. When the sample size equals 100
vehicles, the cumulative frequency and cumulative
percent are the same.
As can be observed from Table 2.1, the exact 50%
and 85% (50th and 85th percentiles) are not found
in the cumulative percent column. To reach these exact percentages, a calculation is completed using
percentages and speeds from the distribution table. Shown below is the equation for calculating speed
percentiles:
Where, SD = speed at PD, PD = percentile desired, Pmax = higher cumulative percent, Pmin = lower cumulative
percent, Smax = higher speed, and Smin = lower speed.
Example speed percentile calculations follow, using the example frequency distribution table in Table 2.1.
The 50th percentile of speed (PD = 50%) falls between 27 and 30 mph (see Table 2.1), So, Smax = 30 mph and
Smin = 27 mph. The higher cumulative percent (Pmax) is 54%, and the lower cumulative percent (Pmin) is 23%.
Therefore, to find SD at PD = 50%,

21. SD =
50%−23%
54% −23%
(30mph 27mph) 27 mph= 29.6 mph.
The 85th percentile of speed (PD = 85%) falls between 33 and 36 mph (see Table 2.1), So, Smax = 36 mph and
Smin = 33 mph. The higher cumulative percent (Pmax) is 86%, and the lower cumulative percent (Pmin) is 72%.
Therefore, to find SD at PD in this case (85th percentile of speed),
SD =
85%−72%
86% −72%
(36mph 33mph) 33 mph= 35.8 mph.
Stopwatch method: The stopwatch method can be used to successfully complete a spot speed study using a
small sample size taken over a relatively short period of time. The stopwatch method is a quick and
inexpensive method for collecting speed data.
Key Steps to a Stopwatch Spot Speed Study: stopwatch spot speed study includes five key steps:
1. Obtain appropriate study length.
2. Select proper location and layout.
3. Record observations on stopwatch spot speed study data form.
4. Calculate vehicle speeds.
5. Generate frequency distribution table and determine speed percentiles.
Obtain Appropriate Study Length: The study length is important because it is used in the calculation of
vehicle speeds. Table 2.3 provides recommended study lengths, which are based on the average speed of the
traffic stream. Using these recommended study lengths makes speed calculations straightforward and less
confusing. If these lengths are not appropriate, another length can be used assuming it is long enough for
reliable observer reaction times.
Select Proper Location and Layout: Figure 2.1
illustrates a typical layout for conducting a spot
speed study using a stopwatch. When selecting a
location and layout, care must be exercised so that
the observer can clearly see any vertical reference
posts. The observer should be positioned higher than the study area and be looking down. The position could
be on a bridge or a roadway back slope. The observer should use reference points to aid in collecting the
elapsed time it takes a vehicle to travel through the study area. The reference point to start timing may be a
brightly colored vertical post. The reference point to end timing may be a tree or a signpost in the observer’s
sight line. An accurate sketch of the site should be
documented, including number of lanes, position of
observer, and description of reference points (see
Figure 2.1 for an example).
Record Observations on Stopwatch Spot Speed
Data Form: On the stopwatch spot speed data form (a
blank form is provided in Appendix A.1), the observer
records the date, location, posted speed limit, weather
conditions, start time, end time, and down time. As the
front wheels of a vehicle (or only the lead vehicle in a group) cross a mark or pavement crack at the beginning
of the predetermined study length, the observer starts the stopwatch. The watch is stopped when the vehicle’s
front wheels pass a reference line in front of the observer. A slash is recorded on the data form corresponding
to the elapsed time observed.
Calculate Vehicle Speeds: To calculate vehicle speed, use the predetermined study length and the elapsed
time it took the vehicle to move through the course (as recorded on the stopwatch data form) in the following
formula (Robertson 1994): V =
𝐷
1.47𝑇
Where, V = spot speed (mph), D = length (feet), and T = elapsed time (seconds). In the equation, 1.47 is a
constant that converts units of feet per second into miles per hour. For example, if the spot speed study length
is 100 feet and the motorist’s elapsed time is 2.5 seconds, the motorist is traveling at
V =
100
1.47 𝑥 2
= 27mph
Generate Frequency Distribution Table and Determine Speed Percentiles: Determine the 50th and 85th
speed percentiles using a frequency distribution table and calculations as described earlier.

22. Solution of table 2.4:
The study shows that the 50th
percentile or median speed falls between 27.2 and 28.9 mph, and the 85th
percentile of speed falls between 33.3 and 35.2 mph. Equation 2.1 is used to find the exact speeds for the 50th
and 85th
percentiles of speed. For the 50th
percentile of speed, PD =50%, Pmax = 54%, Pmin = 41%, Smax = 28.9
mph, and Smin = 27.2 mph, so
SD =
50%−41%
54% −41%
(28.9mph 27.2mph) 27.2 mph= 28.4 mph.
For the 85th
percentile of speed, PD = 85%, Pmax = 92%, Pmin = 83%, Smax = 35.2 mph, and Smin =
33.3 mph, so
SD =
85%−83%
92% −83%
(35.2mph 33.3mph) 33.3 mph= 33.4 mph.
Radar Meter Method: A radar meter is a commonly used device for directly measuring speeds in spot speed
studies (see Figure 2.4). This device may be hand-held, mounted in a vehicle, or mounted on a tripod. The
effective measuring distance for radar meters ranges from 200 feet up to 2 miles (Parma 2001). A radar meter
requires line-of-sight to accurately measure speed and is easily operated by one person. If traffic is heavy or
the sampling strategy is complex, two radar units may be needed.
Key Steps to a Radar Metter Spot Speed Study: A radar meter spot speed study includes four key steps:
1. Select proper location and placement of radar meter.
2. Determine an appropriate selection strategy.
3. Record observations on radar meter spot speed study data form.
4. Generate frequency distribution table and determine speed percentiles.
Select Proper Location and Placement of Radar Meter: Proper placement of the radar meter at the study
area is critical. The positioning of the radar unit is determined by the capabilities of the radar unit (as listed in
the users’ manual). The unit should also be concealed from the view of motorists. Effective ranges may be up
to 2 miles, but as the distance increases the effectiveness decreases (Robertson 1994). The least accurate
position, which often results in no readings at all, is obtained when the meter is aimed at a 90-degree angle to
the roadway centerline (Homburger et al. 1996). An accurate sketch of the site should be documented,
including number of lanes, position of observer, and description of reference points.
Determine an Appropriate Selection Strategy: Except for studies conducted under low-volume conditions,
it is impossible to obtain a radar measurement for every vehicle. For peak flow analysis, speeds are measured
during the peak period. For assessing general speed trends or for setting speed limits, off-peak measurements
are more appropriate.
The selection of the target vehicle that represents the vehicle population under study is also important. A good
question to ask is, “What type or types of vehicles are of concern—cars, trucks, buses, or others?” Typically
cars, station wagons, pickup and panel trucks, and motorcycles are classified as passenger cars. Other trucks

23. and buses are classified as trucks. School buses and farm equipment may be recorded separately. When the
target vehicle is defined, a selection strategy is developed to provide a random sample. A random sample will
reduce the tendency to select the vehicles that stand out. For example, the observer could obtain a speed
reading from every fourth vehicle or every tenth vehicle.
Record Observations on Radar Meter Spot Speed Data Form: On the radar meter spot speed data form (a
blank form is provided in Appendix A.2), the observer records the date, location, posted speed limit, weather
conditions, start time, end time, and down time. A slash is recorded on the data form corresponding to speed
observed for each selected vehicle (or only the lead vehicle in a group) under the appropriate vehicle-type
classification.
Generate Frequency Distribution Table and Determine Speed Percentiles: Determine the 50th and 85th
speed percentiles using a frequency distribution table and calculations as described earlier.
The study shows the 50th
percentile or median speed was
between 34 and 35 mph, and the 85th
percentile of speed
was between 40 and 41 mph. Equation 2.1 is used to find
the exact speeds for the 50th
percentile of speed and the
85th
percentile of speed. For the 50th percentile of speed,
PD = 50%, Pmax = 53%, Pmin = 45%, Smax = 35 mph, and Smin = 34 mph, so
SD =
50%−45%
53% −45%
(35mph 34mph) 34 mph= 34.6 mph.
For the 85th
percentile of speed, PD = 85%, Pmax = 88%, Pmin = 81%, Smax = 41 mph, and Smin =
40 mph, so
SD =
85%−81%
88% −81%
(41mph 40mph) 40 mph= 40.6 mph.
Pneumatic Road Tube Method: The pneumatic road tube method is normally used for longer data collection
time periods than those of either the stopwatch or radar meter method. Using this method, pneumatic tubes
are placed in the travel lanes (see Figure 2.7) and are connected to recorders located at the side of the road.
Key Steps to a Pneumatic Road Tube Spot Speed Study: A pneumatic road tube spot speed study includes
four key steps (Robertson 1994):
1. Perform necessary office preparations.
2. Deploy and calibrate data collection equipment.
3. Check data and retrieve equipment.
4. Generate frequency distribution table and determine speed percentiles.

24. Perform Necessary Office Preparations: During office preparations, coordinate all data collection activities
with appropriate state and local officials, including transportation, traffic, and law enforcement agencies. For
example, you may coordinate with state or local officials in obtaining traffic control for the deployment and
recovery of equipment. The field team must be briefed on the data collection process to ensure that all
observers are collecting the same type of data. The team should assemble and inspect all tools, supplies, and
equipment. Each piece of equipment should be tested in advance of using.
Deploy and Calibrate Data Collection Equipment: The road tubes are prepared on the roadside to minimize
the time each traffic lane is closed. Workers then place the road tubes across the lanes. The location of the
tubes should be outside the influence of other factors such as an intersection, major access points, etc. The
separation of the pneumatic tubes should be 2–15 feet. For the specific spacing of the pneumatic tubes refer
to the users’ manual. Traffic control should be provided to protect the crew. After placing, the crew should
make sure that the tubes are functioning properly. Finally, the crew can secure the road tubes to the pavement.
To avoid theft, the recorder should be secured.
Check Data and Retrieve Equipment: The accuracy of the equipment in measuring the speeds of the traffic
stream should be checked. The recorder first measures the elapsed time it takes the vehicle to pass over the
tubes. Then this time interval is converted to the corresponding spot speed. The elapsed time can be checked
with a stopwatch. The crew can adjust the recorder until the correct speeds are being recorded. It is advisable
to check the function and accuracy of the equipment at least once during every 24-hour data collection period.
When the data collection period has ended, the recorded data should be checked again for accuracy. Crews
recover data collection equipment by reversing the process they used to deploy it.
Generate Frequency Distribution Table and Determine Speed Percentiles: Determine the 50th
and 85th
speed percentiles using a frequency distribution table and calculations as described earlier.
Q. The accompanying data (TableQ2) shows spot speeds collected at Jalan Duta, KualaLumpur. Based
on statistical method, determine the values of the following:
i) Arithmetic mean speed
ii) Mode speed
iii) Median speed
iv) Standard Deviation
Solution:
Speed
class
(km/hr)
No of
vehicles,
f
Mean
speed,
V
V2
f.v (f.v2
)
10 –14.9 2 12.45 155.003 24.9 310.005
15 –19.9 6 17.45 304.503 104.7 1827.015
20 –24.9 5 22.45 504.003 112.25 2520.013
25 –29.9 4 27.45 753.503 109.8 3014.01
30 –34.9 7 32.45 1053.003 227.15 7371.018
35 –39.9 1 37.45 1402.503 37.45 1402.503
40 –44.9 8 42.45 1802.003 339.6 14416.02
45 –49.9 9 47.45 2251.503 427.05 20263.52
50 –54.9 5 52.45 2751.003 262.25 13755.01
55 –59.9 6 57.45 3300.503 344.7 19803.02
60 –64.9 4 62.45 3900.003 249.8 15600.01
57 18177.52 2239.65 100282.1
Speed Class
(km/hr)
No of vehicles
10 –14.9 2
15 –19.9 6
20 –24.9 5
25 –29.9 4
30 –34.9 7
35 –39.9 1
40 –44.9 8
45 –49.9 9
50 –54.9 5
55 –59.9 6
60 –64.9 4

25. Speed
Running speed: Running speed is the average speed
maintained over a particular course while the vehicle
is moving and is found by dividing the length of the
course by the time duration the vehicle was in motion.
So, Running speed = Length of course / Running time
= Length of course / (Journey time- Delays)
Journey speed: Journey speed is the effective speed of the vehicle on a journey between two points and is
the distance between the two points divided by the total time taken for the vehicle to complete the journey
including any stopped time. If the journey speed is less than running speed, it indicates that the journey follows
a stop-go condition with enforced acceleration and deceleration. The spot speed here may vary from zero to
some maximum in excess of the running speed. A uniformity between journey and running speeds denotes
comfortable travel conditions.
So, Journey speed = Distance/ Total journey Time (including Delays).
Time Mean Speed (Arithmetic mean) and Space Mean Speed (Harmonic mean): Time mean speed is
defined as the average speed of all the vehicles passing a point on a highway over some specified time period.
Space mean speed is defined as the average speed of all the vehicles occupying a given section of a highway
over some specified time period. Both mean speeds will always be different from each other except in the
unlikely event that all vehicles are travelling at the same speed. Time mean speed is a point measurement
while space mean speed is a measure relating to length of highway or lane, i.e. the mean speed of vehicles
over a period of time at a point in space is time mean speed and the mean speed over a space at a given instant
is the space mean speed.
Difference between TMS and SMS:
Use of Spot Speed Study:
•To use in planning traffic control and traffic regulation.
•To use in geometric design for redesigning the existing highways.
•To use the accident studies.
•To study the traffic capacity.
Capacity: Capacity is defined as the maximum number of vehicles, passengers, or the like, per unit time,
which can be accommodated under given conditions with a reasonable expectation of occurrence.
Speed and Delay study:
•The speed and delay studies give the running speeds, overall speeds, fluctuations in speeds, and the delay
between two stations of a road.
•It gives information such as the amount, location, duration, and cause of delay in the traffic stream.
•The result of the spot and delay studies are useful in detecting the spot of congestion.
•The delay or time lost traffic during the travel period may be either due to fixed delays or operational delays.
•Fixed delay occurs primarily at intersections due to traffic signals and at level Crossings.
•Operational delays are caused by the interference of traffic movement, such as turning vehicles, parking
vehicles, pedestrians, etc.
Various methods to carry out speed and delay survey are:
1. Floating car method or Riding check method
2. License plate record method or Vehicle number method
3. By interview
4. By photography and videography
License Plate Method: It synchronized stop watches or voice recording equipment are used. Observers are
stationed at the entrance and exit of a test section where information of travel time is required. The timings
and vehicle numbers are noted by the observers of the selected sample. From the office computations, travel
time of each vehicle could be found. But the method does not give important detail such as causes of delays
and the duration and number of delays within the test section.
Interview Technique: The work can be completed in a short time by interviewing and collecting details from
the road user spot. However the data collected may not provide with all the details correctly.
Elevated observation and Photographic Technique: This are useful for studying short test section like
intersection.

27. Traffic Volume Study
Traffic Volume: It 3
is the number of vehicles crossing a section of road per unit time at any selected period.
It is used as a quantity measure of flow: the commonly units are vehicles/day or vehicles/hour.
Object and Uses of Traffic Volume Study:
•It is generally accepted as a true measure of the relative importance of roads and in deciding the priority for
improvement and expansion.
•It is used in planning, traffic operation and control of existing facilities and also for planning the new facilities.
•It is used in the analysis of traffic patterns and trends.
•Useful in structural design of pavement.
•Pedestrian traffic volume study is used for planning side walk, cross walks, subway and pedestrian signals.
Scope of Traffic Volume Study:
1.Design
2.Planning
3.Improvement
4.Dynamic Traffic Management
5.Estimating Highway Use
6.Computing Accident Rates
7.Traffic Stream
Main Methods of Counting:
Manual Counting Method: 1. Direct Method and 2. Indirect Method
Automatic Counting Method: 1. Contact System and 2. Contact-less System.
Manual Counting Method: A manual classified count (MCC) involves counting all the vehicles passing a
selected location on a road for a pre-determined period of time. The count is conducted by persons standing
at the roadside and recording passing vehicles on a form, hence the term “manual traffic count”. This
distinguishes it from counts by machines that can record passing vehicles automatically, which are known as
“automatic traffic counts”. The count records individual vehicles by categories (i.e. a truck or car) and the
direction they are travelling in. This is the reason it is called a ‘classified count’.
Manual Method Steps:
Employs a field team to count and classify traffic volume.
No. of observers required depends upon no. of Lanes and type of information required.
More desirable to record traffic in each direction of travel by posting separate observers for each direction.
For all-day counts, work in three shifts can be arranged.
Data is recorded by Five- dash system. (IIII)
Equipment for manual counts: •Watch, •clip board, •Pencil, rubber, sharpener, •Data sheet
Advantage:
Details such as vehicle classification and no. of occupants can be obtained.
Records turning movement data of vehicles at intersections.
Analyzing traffic characteristics in unusual conditions such as adverse weather conditions, traffic
breakdowns, temporary closure of any lane of highway etc.
Comparatively cheaper method of counting.
Data accumulated by manual methods are easy to analyze.
Disadvantage:
Not practicable to have manual count for 24 hrs of the day and throughout the year.
Counts of remote areas is possible by automatic devices.
Automatic Counting Method: An automatic survey involves placing a tube or loop across a road which is
connected to a box containing the means for storing the information. In this method, vehicles are counted
automatically without any human involvement. The automatic count method provides a means for gathering
large amounts of traffic data. Automatic counts are usually taken in 1-hour intervals for each 24-hour period.
The counts may extend for a week, month, or year. When the counts are recorded for each 24-hour time period,
the peak flow period can be identified.
There are two techniques of automatic counting:
a) Contact system based on pneumatic, mechanical, magnetic or
b) Contact less system ultrasound/infrared radar, based on micro wave, electrical/optical, CCTV/video image
processing method etc.

28. Types: Automatic counts are recorded using one of three methods:
(a) Portable counters, (b) Permanent counters, and (c) Videotape
Portable Counters: Portable counting is a form of manual observation. Portable counters serve the same
purpose as manual counts but with automatic counting equipment. The period of data collection using this
method is usually longer than when using manual counts. The portable counter method is mainly used for 24-
hour counts. Pneumatic road tubes are used to conduct this method of automatic counts Specific information
pertaining to pneumatic road tubes can be found in the users’ manual.
Permanent Counters: Permanent counters are used when long-term counts are to be conducted. The counts
could be performed every day for a year or more. The data collected may be used to monitor and evaluate
traffic volumes and trends over a long period of time.
Videotape or Videography: Observers can record count data by videotaping traffic. Traffic volumes can be
counted by viewing videotapes recorded with a camera at a collection site. A digital clock in the video image
can prove useful in noting time intervals.
Pneumatic road tube: Pneumatic road tube sensors send a burst of air pressure along a rubber tube when a
vehicle’s tires pass over the tube.
Bending plate: A weight pad attached to a metal plate embedded in the road to measure axel weight and
speed. It is an expensive device and requires alteration to the road bed.
Equipment Used For Automatic Counting Method:
1. Pneumatic Tubes 2. Weigh-In-Motion Sensor Types 3. Vehicles counting by video camera
4. Traffic counters 5. Bending plate and pneumatic road tube 6. Automatic traffic recorders
Factors affecting vehicle counting:
❖Weather conditions, ❖Purpose of the traffic counting, ❖Method of traffic counting.
❖Location of the counting sites, ❖Traffic flow level, ❖Road type, ❖Traffic composition.
Annual Average Daily Traffic (AADT): The total yearly volume divided by number of days in the year.
Average Daily Traffic(ADT): The total volume during a given time period, in whole days greater than one
day and less than one year, divided by number of days in that time period.
Design Hourly Volume: It is the economic hourly flow of future year, which is used for designing geometric
features of roadway. It is chosen in such a way that during the design period it should not be exceeded too
often or too much.
Passenger Car Unit: The different vehicle classes have a wide range of statics characteristics and dynamic
characteristics, apart from these the driver behavior of the different vehicle classes is also found to vary
considerable. Therefore mixed traffic flow characteristics are very much complex when compare to
homogeneous traffic and it is difficult to estimate the traffic volume, capacity of roadway under the mixed
traffic flow, unless the different vehicle classes are converted to one common standard vehicle unit.
Therefore it is a common practice to consider the passenger car as the standard vehicle unit to convert the
other vehicle classes and this unit is called passenger car unit PCU.
Factor affecting Passenger Car Unit:
❑Vehicle characteristics
❑Speed distribution of the mixed
traffic stream, volume to capacity ratio.
❑Roadway characteristics.
❑Regulation and control of traffic.
❑Environmental and climatic
conditions.

29. Traffic Control Device
Traffic control device: Traffic control device is the medium used for communicating between Traffic
engineer and road users. Unlike other modes of transportation, there is no control on the drivers using the
road. Here Traffic control devices comes to the help of the Traffic engineer. The major types of Traffic control
devices used are- Traffic signs, road markings, Traffic signals and parking control.
The various aids and devices used to control, regulate and guide traffic are:
(a) Signs (b) Signals (c) Markings (d) Islands
Requirements of a Traffic Control Device:
1. The control device should fulfill a need: Each device must have a specific purpose for the safe and
efficient operation of traffic ow. The superfluous devices should not be used.
2. It should command attention from the road users: This affects the design of signs. For commanding
attention, proper visibility should be there. Also the sign should be distinctive and clear. The sign should be
placed in such a way that the driver requires no extra effort to see the sign.
3. It should convey a clear, simple meaning: Clarity and simplicity of message is essential for the driver to
properly understand the meaning in short time. The use of color, shape and legend as codes becomes important
in this regard. The legend should be kept short and simple so that even a less educated driver could understand
the message in less time.
4. Road users must respect the signs: Respect is commanded only when the drivers are conditioned to expect
that all devices carry meaningful and important messages. Overuse, misuse and confusing messages of devices
tends the drivers to ignore them.
5. The control device should provide adequate time for proper response from the road users: This is
again related to the design aspect of traffic control devices. The sign boards should be placed at a distance
such that the driver could see it and gets sufficient time to respond to the situation. For example, the STOP
sign which is always placed at the stop line of the intersection should be visible for at least one safe stopping
sight distance away from the stop line.
General Color Coding:
Yellow – general warning
Red – stop or prohibition
Green – Direction guidance
Orange – construction and maintenance warning
Sign:
Types: As per IRC, there are three types of traffic signs:-
1) Regulatory or Mandatory signs 2) Warning signs and 3) Informatory signs
Regulatory signs: These signs require the driver to obey the signs for the safety of other road users. These
are meant to inform the road users of certain laws, regulations and prohibitions, the violation of the signs is a
legal offence. These signs have generally black legend on a white background. They are circular in shape with
red borders.
The regulatory signs can be further classified into:
1. Right of way series: These include two unique signs that assign the
right of way to the selected approaches of an intersection. They are the
STOP sign and GIVE WAY sign For example, when one minor road
and major road meets at an intersection, preference should be given to
the vehicles passing through the major road. Hence the give way sign
board will be placed on the minor road to inform the driver on the
minor road that he should give way for the vehicles on the major road.
In case two major roads are meeting, then the traffic engineer decides
based on the traffic on which approach the sign board has to be placed.
Stop sign is another example of regulatory signs that comes in right of
way series which requires the driver to stop the vehicle at the stop line.
2. Speed series: Number of speed signs may be used to limit the speed of the vehicle on the road. They include
typical speed limit signs, truck speed, minimum speed signs etc. Speed limit signs are placed into limit the
speed of the vehicle to a particular speed for many reasons. Separate truck speed limits are applied on high
speed roadways where heavy commercial vehicles must be limited to slower speeds than passenger cars for
safety reasons. Minimum speed limits are applied on high speed roads like expressways, freeways etc. where

30. safety is again a predominant reason. Very slow vehicles may present hazard to themselves and other vehicles
also.
3. Movement series: They contain a number of signs that affect specific vehicle maneuvers. These include
turn signs, alignment signs, exclusion signs, one-way signs etc. Turn signs include turn prohibitions and lane
use control signs. Lane use signs make use of arrows to specify the movements which all vehicles in the lane
must take. Turn signs are used to safely accommodate turns in no signalized intersections.
4. Parking series: They include parking signs which indicate not only parking prohibitions or restrictions, but
also indicate places where parking is permitted, the type of vehicle to be parked, duration for parking etc.
5. Pedestrian series: They include both legend and symbol signs. These signs are meant for the safety of
pedestrians and include signs indicating pedestrian only roads, pedestrian crossing sites etc.
6. Miscellaneous: Wide variety of signs that are included in this category are: a "KEEP OF MEDIAN" sign,
signs indicating road closures, signs restricting vehicles carrying hazardous cargo or substances, signs
indicating vehicle weight limitations etc.
Warning signs: These signs are for the safety of on self who is driving and advice
the drivers to obey these signs. Warning signs are used to warn the road users of
certain hazardous conditions that exists on the roadway.
These signs are in the shape of equilateral triangle. They are having white
background, red border and black symbols. The side of triangle is 45cm.
Informative signs: These signs provide
information to the driver about the facilities
available ahead, and the route and distance to reach
the specific destinations to make travel easier, safe
and pleasant.
Various Informatory signs are:
i. Direction and place identification signs,
ii. Facility information signs,
iii. Parking signs,
iv. iv. Flood gauge
Signal: These are provided at road intersections. These are control devices which could alternatively direct
the traffic to stop proceed at intersections using red and green light signals automatically. The main
requirement of traffic signals are to draw attention.
Advantage:
1. Maintain orderly flow of traffic.
2. Reduce certain types of accidents.
3. Improves safety and efficiency of movement of vehicles.
4. Pedestrians can cross the roads safety.
5. More economical than manual control.
6. Increase in speed along the major road traffic.
Disadvantage:
The rear and end collision may increase.
Improper design and location of signals may lead to violations the control systems.
Failure of the signal due to electrical power failure or any other defects may cause confusion to the road
users.
Types of Traffic Signals:
Fixed time signals
Manually operated signals
Marking: These are the lines, patterns, words of other devices for control, warning and guidance or
information of road users. The markings serve as a psychological barrier and signify the delineation of
traffic path and its lateral clearance from traffic hazards for the safe movement of traffic. The road markings
are classified as longitudinal markings, transverse markings, object markings, word messages, marking for
parking, marking at hazardous locations etc.
Various types of markings are:-
1. Pavement markings, 2.Kerb markings, 3.Object markings, 4.Reflector markings

31. Pavement Marking: These are generally be of white paint.
Yellow color marking are used to indicate parking restrictions.
Kerb Marking: Marking on the kerb indicate certain
regulations like parking regulations. Marking on the kerb and
edges of islands with alternative black and white line increase
the visibility from a long distance.
Object Marking: Physical obstruction on or near the roadway
are hazardous and hence should be properly marked. Typical
obstructions are signs and signals, traffic islands, narrow
bridges, culvert.
Reflector Marking: These are used as hazard markers and
guide markers for safe driving during night. These markers should be visible from a long distance of about
150 m.
Traffic Characteristics
Study of traffic characteristics include: (a) Vehicular characteristics (b) Road user's characteristics
Vehicular Characteristics: It would be impracticable and uneconomical if roadway is designed to
accommodate all types of vehicles. Generally, roadway is designed for design vehicle that will enjoy safe
driving and full level of service (LOS) whereas extreme users will get lower safety and LOS.
A Vehicle that represents the design standards is termed as design vehicle whose properties are fixed based
on statistical analysis of population – 50th
percentile (mean) value; 85th
percentile or maximum value etc.
The various vehicular characteristics affecting the road design can be classified as:
(a) Static characteristic and (b) Dynamic characteristics
Static/physical Characteristics:
Axle Load: Weight of vehicle affects structural design of pavement –number of layers, thickness of each
layer, material requirements etc. Bridge/fly-over/grade separator. Fixing gradient of roads in rolling terrain,
ramp, bridge approach etc. and it depends on Vehicles weight/power ratio.
Width: Width of vehicle affects: (a) Lane width, (b) Shoulder width (c) Width of parking bay
Length: Length of vehicle affects: (a) Design of horizontal alignment i.e. determination of extra widening
and minimum turning radius, (b) Passing sight distance, (c) Parking facility (bay & isle), (d) Roadway capacity
Height: Height of vehicle affects: (a) Clearance to be provided under the structures e.g. under-bridge, over-
bridge, signpost gantry, electric services lines etc.
Dynamic Characteristics:
(a) Speed - It is such a factor that controls most of the geometric standards of the highway viz.
(b) Horizontal and vertical alignment design
(c) Limiting radius
(d) Grade
(e) Length of transition curve
(f) Super-elevation
(g) Stopping sight distance
(h) Width of the pavement of straight and curves
(i) Capacity/LOS
(j) Intersection design and control
(k) Skid resistance.
Road User's Characteristics:
Road users e.g. pedestrians and drivers are most complex and least understood element of roadway system.
As road users are a major part of the system, human limitations and behavior must be understood and taken
into account in all traffic engineering and design maters.
It has been found that over 90 per cent of all highway accidents are due to errors of judgment.
As such it is of paramount importance for a traffic engineer to study the factors which directly or indirectly
influence the manner and rate at which errors occur.
The behavior of a rood user depends upon the factor like education, environment and training.
's driving education/road sense can be improved by strict licensing/road safety education policy.

32. As for pedestrians there is no scope for formal education and they are most exposed vulnerable group
comprising largest road users group especially within urban area - they need protection by law. Priority on
right-of-way is given to pedestrian.
Therefore, it is the driver's responsibility for safety of pedestrian and as such his behavior in this 1.egard is
very important.
Factors affecting the road user characteristics: The factors which affect behavior of road users are:
Physical factors: (a) Eye-sight/vision, (b) Hearing power, (c) Other defects/.disabilities (d) Fatigue, alcohol
or drugs, illness etc. are the temporary physical characteristics which affect the reaction time, judgment
time, alertness
Mental factor: (a) Intelligence, (b) Education, skill, knowledge, (c) Experience/training of the drivers
Psychological: (a) Impatience, (b) Attentiveness, (c) ability of follow regulation and (d) Maturity.
Types of Flow/Volume:
Interrupted flow: Flow at stopped and go situation.
Uninterrupted flow: When the flow is smooth.
Saturation flow: The maximum hourly rate of an approach at a signalized junction.
Service flow rate: The maximum hourly rate of a roadway section during a given period under prevailing
roadway condition.
Free flow: When drivers face no restriction in driving and can maintain their desired speeds.
Forced Flow: When lane changing opportunity decreases with increasing traffic volume and drivers are forced
to follow slow leaders.
Stable/steady flow: When demand in well below the roadway capacity and the average rate of flow remains
almost constant with time.
Unstable flow: When demand is at or near or exceed the roadway capacity and the average rate of flow
fluctuates largely with time.
Peak flow: Flow at peak period.
Contra flow: For repair works, an arrangement on a large road by which traffic going in both direction uses
only one side of the road.
Induced flow: Flow that is generated because of new or improved roadway facility.
Traffic flow characteristics and studies
Traffic Flow (q): The rate at which vehicles pass at a fix point (vehicles per hour) = N (3600/t).
Traffic Density (k): No. of vehicles (N) over a stretch of roadway (L) i.e. vehicles per kilometer =N/L
Time headway: Time interval between the passages of the fronts of the successive vehicles at a specified point.
Average time headway = average travel time per unit distance X average space headway
Space headway: Distance between fronts of successive vehicles.
Average space headway= space mean speed X average time headway
Flow Density Relationship:
Flow = density X space mean speed, so, q= K x V
Density = 1/ space headway = 1/hs
Space mean speed = flow X Space headway = q x hs
Density = flow X time per unit distance, so, K = q x t
Traffic capacity studies:
Traffic capacity: The ability of a roadway to accommodate traffic volume. It is expressed as the maximum
number of vehicle in a lane or a road that can pass a given point in unit time, usually an hour.
Volume represent an actual rate of flow whereas capacity indicates a maximum rate of flow with a certain
level of service.
Basic capacity: It is the maximum no. of passenger car that can be pass a given point on a roadway during
one hour under the most nearly ideal roadway and traffic conditions. It is otherwise known as theoretical
capacity.
Possible capacity: It is the maximum no. vehicle that can pass a given point on a roadway during one hour
under prevailing roadway and traffic conditions.
Practical capacity: It is the maximum no. of vehicle that can pass a given point on a roadway during one
hour, without traffic density being so great as to cause unreasonable delay, hazard or restriction to the driver
freedom to maneuver under the prevailing roadway and traffic conditions.
C = 1000 V/S here, S= average spacing of vehicle in m, C is the capacity in vehicle per hour per lane.

33. Thirtieth highest hourly volume or the design hourly volume: It is found from the plot between hourly
volume and the number of hours in a year that the traffic volume is exceeded. The 30th highest hourly volume
is the hourly volume that will be exceeded only 29times in a year and all other hourly volumes of the years
will be less than this volume. The 30th highest traffic volume is found to be satisfactory from both facility and
economic considerations.
Peak-Hour Factor: It is basically represent the variation in traffic flow within an hour. Observations of traffic
flow consistently indicate that the flow rates are found in the peak. A 15 minute period within an hour is not
sustained throughout the entire period and that is why we need to use the peak-hour factor. Normally on
freeways the peak-hour factor values range from 0.80 to 0.95.
DDHV/PHV: Directional Design Hourly volume (DDHV) ~ Peak Hourly Volume
DDHV (vehicle/hour) = AADT x K x D
Where, AADT = Average Annual Daily Traffic (vehicle/ day)
K = Proportion of daily traffic occurring in peak hour (decimal)
D = Proportion of peak hour traffic traveling in the peak direction (decimal)
For design purpose k factor is generally chosen as 30HV and D factor as the percentage of traffic in
predominant direction during the design year. General ranges for K and D factors are as follows:
Facility Type K Factor D Factor
Rural 0.15– 0.25 0.65– 0.80
Suburban 0.12– 0.15 0.55– 0.65
Urban 0.07– 0.12 0.50– 0.60
Sub-Hourly Volume: Sub-hourly Volume ~ 15-min Volume
Suppose, the peak 15-min volume observed = 750 vehicle.
So, the Hourly Volume = 15-min volume x4 =750 x4 = 3000 vehicle/hour
Q. Data collected are as follows:
Time Interval Volume ( veh)
4:00-4:15pm 950
4:15-4:30pm 1100
4:30- 4:45 pm 1200
4:45-5:00pm 1050
Hourly Volume 4300
Solution:
We find from the table, HV = 4300 vehicle/hour V15 = 1200 vehicle
Therefore, (15min por por data niya hoyase, tai 4 diye divide kora lagbe)
PHF =
𝐻𝑉
4 𝑥 𝑉15
=
4300
4 𝑥 1200
= 0.90
Peak Hour Factor: For freeway/expressway: (5min por por data niya hoyase, tai 12 diye divide kora lagbe)
PHF =
𝐻𝑉
12 𝑥 𝑉15
Where, HV = Hourly volume (vehicle/hour) and, V5 = max. 5-min volume within the hour (vehicle)
Traffic Capacity: Maximum flow occurs when the speed is Vsf/2 and the density is Kj/2 -
qmax =
𝑉𝑠𝑓 𝑋 𝐾𝑗
4
Where, Kj = jam density= 1000/spacing of vehicle
Vsf = free mean speed
K= q/v
q = average volume of vehicle,(vehicle/hr)
V= space mean speed of vehicle, kmph

34. Level of service (LOS): It is define as a qualitative measure describing the operational condition within a
traffic stream, and their perception by motorist and passengers.
Or Rating of acceptable level of congestion.
LOS design types and their property:
LOS-A: Free flow, low traffic, high speed
LOS-B: Stable flow, noticeable traffic
LOS-C: Stable flow, traffic interactions,
LOS-D: Unstable flow, High density, movement restrictions
LOS-E: Unstable flow, lower speed, volume is nearly equal to capacity, little freedom
LOS-F: Unstable flow, no freedom, traffic volume can drop to zero, stop & go
In India, as per IRC:
LOS-B: For the design of Rural Roads
LOS-C: For the design of Urban Roads

36. Valley Curve mathematics problems: