Traffic engineering speed ,density volume relations

1. Speed-Flow-Density relation
By
P.Umasai Krishna M.tech,.(Ph.D)

2. Speed
Space mean speed: Avg speed of all vehicles occupying a given
section of a highway over some specified time.
Vs= N/(⅀1/Vi)
Time mean speed: Avg instantaneous speed of observed
vehicles at the spot
Vt= ⅀Vi/n
Traffic Volume or flow:
No of vehicles per unit time
Q=N/T

3. Density (k): No of vehicles per unit length of the road at that
instant
The vehicle density of a road is expressed as the number of
vehicles per lane-Km of the road.
Relation b/w speed, density and volume:
Q=K * Vs
Time head way: Time gap between two successive vehicles
which are crossing a section on the road
Ht =3600/q
Space head way: The avg distance b/w successive vehicles
or inverse of density.
S=1/k =1000/k

4. Speed and travel time:
 As the travel speed increases the travel time to cover a
distance decreases.
The travel time per unit of a road length is inversely
proportional to the speed.
T=1/V hr/km
=60/v min/km
=3600/V sec/km

6. Capacity flow
 The max speed value is called free mean speed
 max density at zero speed is called jam density.
 The maximum flow occurs when the speed is Vsf/2,
and density is Kj/2 and therefore
Qmax = Vsf * Kj/4
Capacity: It is the ability of the roadway to allow maximum traffic
flow.
C=1000 V/S
It is expressed as the max number of vehicles in a lane that can pass a
given point.
Basic capacity: It is the max number of passenger cars that can pass a
given point on a lane during one hour under nearly ideal roadway
and traffic conditions which can be possibly be attained. It is also
called theoretical capacity.
S=0.278Vt+L

7. Possible capacity:
It is the max number of vehicles that can a pass a given point on a
lane during one hour under prevailing roadway and traffic
condition.
The possible capacity of a road is generally less than the basic
capacity.
When the prevailing condition is bad then possible capacity is
equal to zero.
When prevailing condition approach to ideal traffic condition
then the possible capacity is equal to basic capacity.
The range of possible capacity is zero to basic capacity.

8. Practical capacity
 It is the max number of vehicles that can pass a given point on a lane during
one hour, without traffic density being so high as to cause unreasonable
delay under prevailing roadway and traffic condition.
 It is also called as design capacity
 S=SSD+L
Problems(Gate)
1. The design speed of a traffic lane is 80kmph.Average length of the vehicle
is 6m.the capacity of the road ,when the reaction time of the driver is
2sec.Coeffiecient of longitudinal friction is 0.35.
Solu: SSD=Vt+V2/2gf =0.278Vt+V2/254f=116m
S=SSD+L=116+6=122m
C=1000v/s =656 veh/hr/lane

9. Factors affecting practical capacity
 Lane width: As the lane width decreases, the capacity decreases.
 Lateral clearance: Vertical obstructions such as retaining walls, raised kerbs,
parked vehicles near the lane reduces the effective width of a lane and thus
result in the capacity of the adjacent lane.
 Width of shoulders: narrow shoulder reduces the effective width of adjacent
traffic lanes as the vehicles travel towards the centre of the pavement.
 Commercial vehicle: It occupies greater space and influence the other traffic
in the same lane which reduces the capacity of the road.
 Alignment: If the alignment and geometrics are not up to the desired
standards the capacity will decrease.
 Road geometrics: Valley curves, steep gradients reduces the speed and
consequently result in lower highway capacity.
 Presence of intersections: Intersections restrict free flow of traffic and thus
adversely affect the capacity.

10. Level of service
 Capacity and Level of service are two related terms.
 Capacity analysis tries to give a clear understanding of how
much traffic a given transportation facility can accommodate.
(Quantitative measure)
 Level of service tries to answer how good is the present traffic
situation on a given facility. (Qualitative measure)
 For a given road or facility, capacity could be constant. But
actual flow will be different for different days and different
times in a day itself. The intention of LOS is to relate the traffic
service quality to a given flow rate of traffic
 Highway capacity manual (HCM) developed by the transportation
research board of USA provides some procedure to determine level
of service. It divides the quality of traffic into six levels ranging form
level A to level F.

11. LOS graph

12. Graph

13. Los for free way segment
LOS Vehicles/km/lan
e
Velocity V/C
Km/hr
A 0-7 120 0.35
B 7-11 120 0.55
C 11-16 114 0.77
D 16-22 99 0.92
E 22-28 85 1
F >28 <85 >1

14. Los for Intersection
LOS Sec/veh at signalised Sec/veh at
unsignalised
Km/hr
A <=10 <=10
B 10-20 10-15
C 20-35 15-25
D 35-55 25-35
E 55-80 35-50
F >80 >50

15. Parking Studies
Need:
 The number of lanes are decided based on the design traffic
flow, all the vehicles do not keep moving during the entire day.
 Some portion of the traffic will need to stop or park at the
desired locations
 For commercial vehicles need space near the destination for
loading/ unloading operations.
 In market areas parking facility for short duration is required.
 Even a few vehicles parked along the road side will reduce the
effective width of the carriage way.

16. Investigation for parking studies
 Study of existing parking characteristics and the facilities
available
 Inventory of additional parking space that could be availed
 Determination of the actual parking demand during periods of
the day
 Planning and design of suitable parking facilities that are
suitable to meet the demand.

17. Types of parking studies
 On street or kerb parking
 Off street parking
On street parking:
Parallel parking
Angle parking
 30 degrees
 45 degrees
 60 degrees
 90 degrees

18. Off street parking
 Surface parking
 Multi-storeyed parking
On-street parking:
 Vehicles are parked along the kerb which may be designed for
parking
 It is suitable place to park the vehicles near the place they wish
to stop.
 Unless kerb parking facility is adequately designed in advance
while planning a new town, it might lead a lot of inconvenience
to moving vehicles and result in road congestion.
 The restricted kerb parking may either be controlled by police
and a specified parking fee is collected.

19. Layouts of different on street parkings

20. Design of parking facilities

21. Off street parking
 In surface parking lots and multi floor parking garages
the basic operation consist of entrance, acceptance,
storage, delivery and exit.
 Some definite space is required in front of the parking
lot for vehicles during acceptance and also for exit
operations. This space is called reservoir area
 Surface parking lots may be convenient where
sufficient space is available at low cost.
 If the vehicle is left by the driver or owners it is called
self parking. If the vehicle operations is carried out by
attendants it is called attendant parking system.

22. Off street parking
 In multi storeyed lots are restored to when the floor
space available is less and it is very costly.
 It is possible to construct multi stored garages to park a
large number of cars at a time.
 The garages may be above or below the ground.
 It is necessary to provide the inter floor travel facility
for the vehicles which may be either by elevators or by
ramps.
 In mechanized garages the elevators may be designed
to move both in vertical and in horizontal directions.