Conen 442 module2: Highway Geometric Design

Module 2:
Highway Geometric
Design
Wael M. ElDessouki, Ph.D.
CONEN 442 Transportation Engineering S2021 ElDessouki
181

Highway Functional
Classification
182

The Concept of Functional Classification:
Highways are classified according to its functionality.
Most trips can be divided into six stages:
1- Main Movement
2- Transitions
3- Distribution
4- Collection
5 - Access
6 - Termination
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CONEN 442 Transportation Engineering S2021
183

In this example each of the six stages is handled by a separate facility
designed specifically for its function, as following:
1- Main Movement
2- Transitions
3- Distribution
4- Collection
5 - Access
6 - Termination
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Freeways / Highways  High Speed
 Limited Access
Arterials & Collectors  Moderate Speed
 Moderate Access
Local Streets &  Low Speed
Neighborhood Streets  High Access

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185
Based on the previous example, we can
summarize our observations on the Functional
Classification as following conclusions:
1- Traffic Volume is at its highest at the top of the
classification.
2- At the top of the classification mobility is very
high & access is limited.
3- At the lowest in the classification access is very
high & mobility is very low.

Design Controls &
Criteria
186

Design Criteria: Traffic Characteristics :
Projection of future Traffic Demand:
This is a very important component in highway design, because we
design for the future demand not just for current demand. We design for
the life span of the facility:
Facility Life Expectancy
Right of Way (ROW) 100 year
Minor Drainage & Base Courses 50 years
Bridges 25-100 years
Pavement Resurfacing 10 years
Pavement Structure(New) 20-30 years
Pavement (Reconstruction ) 5-10 years
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Speed:
Speed is a key design control for highway geometric design.
Why?
 It defines key design elements in highways such as horizontal
and vertical curves.
 It plays a key role in determining lane capacity and highway
facilities in general.
 Speed is a key factor in functional classification of highway
and consequently affects driver’s selection of the road.
Types of Speed:
Operating Speed:
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Design Criteria: Traffic Characteristics:
Speed:
Types of Speed:
Operating Speed:
Defined as the speed at which drivers are observed operating their
vehicles during free flow condition. The 85th% of observed spot
speed sample can be considered as the Operating Speed.
Design Speed:
Design speed is a selected speed value that is used to determine various
geometric design features of the highway.
The Design Speed must be logical with respect to:
Topography, Anticipated Operating Speed,
 Adjacent land use The functional Classification of the highway
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Speed:
Types of Speed:
Design Speed (cont.):
Design speed should be selected as high as possible
with consideration for:
Desired degree of safety
Mobility
Efficiency
The selected Design Speed Should be consistent with
the speed drivers are likely to expect on a given
facility.
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Speed:
Recommended Ranges for Design Speed:
120 km/h  Rural freeways, expressways & other arterial highways
100 km/h  Suburban freeways & expressways
80 to 110 km/h  Suburban elevated & depressed freeways
50 to 100 Urban arterial streets
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Design Criteria: Highway Capacity
Guidelines for the Selection of Design Level of Service (LOS):
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Functional
Class
Rural
Level
Rural
Rolling
Rural
Mountainous
Urban &
Suburban
Freeway B B C C
Arterial B B C C
Collector C C D D
Local Street D D D D

Highway
Horizontal
Alignment
193

Horizontal Alignment:
Horizontal Alignment
Consists of Tangents
(Straight Segments)
connected by
Horizontal Curves
Types of Horizontal Curves:
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Layout of Simple Horizontal Curve:
 
)
2
/
sin(
*
2
)
(
*
)
(
1
)
2
/
cos(
1
)
2
/
cos(
1
)
2
/
tan(


















R
length
cord
L
R
length
curve
L
R
E
R
M
R
T
c
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Layout of Simple Horizontal Curve:
Curve severity is defined by Radius (R) and the Degree of curvature
(D)
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196
30 m
)
1719


(
in
curvature
of
degree
the
is
D
meters
in
radius
the
is
R
where
R
D

Design of Horizontal Curve:
On horizontal curves, centrifugal force tend to push the vehicle in the
radial direction, the vehicle is maintained by side friction on the
pavement surface and pavement super elevation. Here is the
relationship:
Where ,
V – Speed (m/s) , R- Curve Radius (m), f- Side Friction
e – Super elevation % , g – Gravitational Acceleration(m/s2)
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197
l
f
e
gR
V

 01
.
0
2

Maximum Superelevation: (e)
The AASHTO range (4%-12%) , but:
1- From a construction perspective a 12 is difficult and 8% can be seen
as the maximum
2- The typical superelevation is 5%
Side-Friction factor: (fl )
The coefficient of side friction, (fl ) is function of vehicle speed and other
factors.
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199

Methodology & Considerations for Selecting Side
Friction (fl )
 Horizontal curves should not be designed based on the maximum
available side friction.
 The centrifugal acceleration due to the curve is counteracted by
side friction and Superelevation
 The centrifugal acceleration felt by the driver & passengers is the
portion contributed by the side friction.
 The ball-bank indicator:
see next slide
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200

Methodology & Considerations for Selecting Side
Friction (fl )
 The ball-bank indicator (similar to aircrafts):
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201
Lateral Acceleration = 0.10
This is the portion carried out
by friction

Design Methodologies: R, e & f
 Method 1: Superelevation (e) & side friction (f )are inversely
proportion to curve radius R in a straight line relationship.
 Method 2: All lateral acc. Is sustained by side friction i.e.
f = fmax , then superelevation is then utilized.
 Method 3: for road-users comfort, Consume maximum superelevation
(emax), then use side friction (f )
 Method 4: Same as Method 3, but using Running Speed instead of
Design Speed
 Method 5: Same as Method 1, but it assumes a non linear relationship
for (e & f ) vs. 1/R
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202

Design Methodologies: Example
For a highway with a design speed 90 km/hr, if the maximum
superelevation was set to be 8%. Design a horizontal curve and
determine a safe turning radius such that the maximum centrifugal
force exerted on passengers is maintained below 0.10g
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Design Methodologies: Tangent -to- Curve Transition
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205
Superelevation Runoff Length:

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206
Superelevation Runoff Length:
206

Design Methodologies: Curve-to-Tangent Transition
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207
Tangent Runout Length:
You may use
:
Lt = 2/3 Lr

Design Methodologies: Locations of Transition Curves
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208
PT
PI
PC

Methods of Attaining Superelevation:
Undivided Multilane Highway
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209
A
Advantages:
Short lr , Min. cut & fill
Disadvantages:
Water drainage for
inner lanes

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210
B
Advantages:
Good water drainage
Disadvantages:
Long lr
Large amount of fill

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211
C
Advantages:
Smooth Transition
Disadvantages:
Long lr
Large amount of cut

Divided Multilane Highway
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212
A- Revolving around Roadway Centerline
Advantages:
Short lr , Min. cut & fill
Disadvantages:
Water drainage for
inner lanes

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213
B- Revolving around inner Roadway Edges
Advantages:
Good for water drainage
Disadvantages:
Large a mounts of Fill material
Long transition curves

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214
C - Revolving around median edge
Advantages:
Best in Terms of Safety ,
good on cut & fill material
Disadvantages:
Average on Water drainage for
inner lanes

Offtracking & Lane Widening for Horizontal Curves
What is Offtracking:
 Offtracking is the characteristic, common to all vehicles,
although much more pronounced with the larger design
vehicles, in which the rear wheels do not follow precisely the
same path as the front wheels when the vehicle negotiates
a horizontal curve or makes a turn.
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Offtracking & Lane Widening for Horizontal Curves
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Derivation of Lane Widening for Horizontal Curves
 Track Width on Curve (U)
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217

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218

 Front Overhang (FA)
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219
 Extra Width Allowance(Z)

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221

Elements of Lane Widening for Horizontal Curves
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222

 Curve Widening(w)
 Minimum (w = 0.60 m)
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223
 Extra Width Allowance(Z)

Horizontal Sight Distance Offset (HSO)
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Highway
Vertical
Alignment
225

Vertical Alignment
 It is composed of Vertical Tangents connected with Vertical Curves
 Design Objective: MINIMIZE CUT AND FILL
 Subject to: Maintain LOS and Capacity (at most 2 level drops)
 Grades: Should be comfortable for passengers & Suitable for Vehicles
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Vertical Alignment:
Vehicle Operating Characteristics on
Grades
 Passenger Cars:
For most cars , the range of 4-5% Grade does not affect performance or
loss in speed
 Trucks:
The effect of grades on truck speed is much more significant.
 HCM Exhibit 23.2 A, is used to estimate the equivalent grade for a series of
composite grades to get an equivalent grade & the overall drop in speed.
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Vertical Alignment: HCM Exhibit 23.2 A
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228
1%
4%
3%
2%
5%
6%
7%
8%
Speed km/hr

Vertical Alignment:
Critical Length of Grade for Design
Using the Maximum Grade is not the governing factor, there are other factors
that must be taken into consideration:
1. Size & power of the design vehicle( truck)
2. Speed at entrance to critical length of grade
3. Drop in speed on the critical length
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Vertical Alignment:
Critical Length of Grade for Design
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Vertical Alignment:
Climbing Lane Design
Climbing lanes are usually added for trucks on two way two lane highways
located in mountainous areas, in order to maintain an acceptable level of
service.
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Vertical Alignment:
Climbing Lane Design: Criteria
The following are the three criteria to Justify a climbing lane:
1. Upgrade traffic flow rate >200 veh/hr
2. Upgrade Truck flow rate > 20 truck/hr
3. One of the following:
I. Speed Drop > 15 km/hr  speed drop alone can justify
II. LOS E or F on the grade
III. LOS Drop two or more levels.
ElDessouki
CONEN 442 Transportation Engineering S2021 232

Vertical Alignment:
Climbing Lane Design: Example
For the given grade segment of a 2 lane highway, determine if a climbing lane is
need or not? if needed, please determine its start and end. Facts: Grade 8%,
Volume = 650 km/hr , % Trucks= 5%, Deign Speed = 90 km/hr.
Solution:
Upgrade Volume = ??
Number of Trucks = ??
From the charts:
Drop in Speed ??
Drop in LOS ??
LOS E or F ??
Using the HCM Truck performance curves to determine Start & End of climbing lane
ElDessouki

Vertical Alignment:
Climbing Lane Design: Start/End
ElDessouki
1%
4%
3%
2%
5%
6%
7%
8%
Speed km/hr
Allowable Drop in Speed
( in this case was 15 km/hr)
Below this line a climbing
lane must be added

Vertical Curves
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Vertical Curves: Geometric
Characteristics
 Vertical curves are not circular, they are in the
shape of a parabola, and they are two types: Crest
& Sag curves.
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236
Key Issue: Safety & Clearance
Key Issue: Clearance & Drainage

Characteristics
 Elements of a Vertical curve:
VPI – Vertical point of intersection
VPC – Vertical point of curvature
VPT – Vertical point of tangent
G1 – Approach grade %
G2 – Departure grade %
L – Length of curve in meters
r - rate of change of grade per unit length
Then the curve equation will be:
Where:
Y(X) – elevation for a point at (x) meters from the VPC
Yo – Elevation of the VPC
b – Approach grade, G1% & ElDessouki
237
o
Y
bX
aX
X
Y 

 2
)
(
L
G
G
r
1
2

L
G
G
a
2
1
2


Characteristics
Example:
For the shown vertical curve. Determine:
1- Stations & Elevations for PVC & PVI
2- STA & Elev for the highest point on the curve.
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PVT 2+431.05
Elev@PVT= 236.62 m
L =270 m
PVC
PVI

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239
Design Criteria:
Stopping Sight Distance (S)
Design Crest Vertical Curve:

Design Crest Vertical Curve:
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240
For h1 =1.08 m & h2 = 0.60 m ,
then we can use the following:

Design of Sage Vertical Curve:
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241
Design Criteria:
1. Headlight Stopping Sight Distance (S)
2. Passenger Comfort
3. Drainage Control
4. General Appearance

Design of Sage Vertical Curve:
ElDessouki
242 Headlight/ Stopping Sight Distance (S)
For Passenger Comfort:
For Drainage:
Rate of Curvature (K) = L/A ≤ 51
For Appearance:
Minimum Length Lmin = 30 A
A = │G2 –G1 │

Undercrossing Clearance:
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243
h1 =2.4 m, h2 = 0.6 m

Undercrossing Clearance:
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244
h1 =2.4 m, h2 = 0.6 m

End of Module 2
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245

Conen 442 module2: Highway Geometric Design

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Conen 442 module2: Highway Geometric Design