Unit-3
Traffic Engineering
Q1) A vehicle travelling at 40 kmph was stopped within 1.8 seconds after the application of the brakes. Determine the average skid resistance.
A1)
Initial speed, u = = 11.11 m/sec
Braking Time, t = 1.8 second
Using the fundamental relation of motion for uniform acceleration/retardation,
v = u + at
Retardation, a = u/t = 11.11/1.8 = 6.17 m/sec2
From the relation, Force F = ma = Wf = Wa/g
Average skid resistance, f = a/g = 6.17/9.88 = 0.63
Q2) A vehicle was stopped in 1.4 second by fully jamming the brakes and the skid marks measured 7.0 m. Determine the average skid resistance.
A2)
Using the fundamental relation of motion for uniform acceleration/retardation
(i) v = u + at, as the final velocity v = 0, u = -at
(ii) v2 – u2 = 2as
s = - =
and therefore a =
Given braking distance L = 7.0 m = s
And braking distance, t = 1.4 sec
Average skid resistance, f = a/g = = 0.729
Q3) Estimate the theoretical of a traffic lane with one way traffic flow at stream speed of 40 kmph. Assume the average space gap between vehicles to follow the relation Sg = 0.278 Vt where V is the speed in kmph, t is the average reaction time = 0.7 second, assume average length of vehicles = 5.0 m.
A3)
V = 40kmph
t = 0.7 second
L = 5.0 m
S = 0.278 Vt + L = 0.278 X 40 X 0.7 + 5.0 = 12.78 m
Theoretical capacity, C = 1000V/S = 1000 X 40 /12.78 = 3130 vehicles/hour/lane
Q4) The average normal flow of traffic on crosses roads A and B during design period is 400 and 250 pcu per hour; the saturation flow values on these roads are estimated at 1250 and 1000 pcu per hour respectively. The all red time required for pedestrian crossing is 12 seconds. Design two phase traffic signal by Webster’s method.
A4)
ya = = 400/1250 = 0.32
yb = = 250/1000 = 0.25
Y = ya + yb = 0.32 + 0.25 = 0.57
L = 2n + R = 2 X 2 + 12 = 16 seconds
Co = =
= 67.4 say 67.5 seconds
Ga = = 0.32/0.57 (67.5 – 16) = 29 seconds
Gb = = 0.25/0.57 (67.5 – 16) = 22.5 seconds
All red time for pedestrians crossing = 12 seconds
Providing Amber time of 10 second each for clearance, total cycle time = 29 + 22.5 + 12 + 4 = 67.5 seconds
Q5) Discuss the various traffic studies and their importance.
A5)
Flow
There are practically two ways of counting the number of vehicles on a road. One is flow or volume, which is defined as the number of vehicles that pass a point on a highway or a given lane or direction of a highway during a specific time interval. The measurement is carried out by counting the number of vehicles, nt, passing a particular point in one lane in a defined period t. Then the flow q expressed in vehicles/hour is given by
q =
Types of volume measurements
Speed Study
Speed is considered as a quality measurement of travel as the drivers and passengers will be concerned more about the speed of the journey than the design aspects of the traffic. It is defined as the rate of motion in distance per unit of time. Mathematically speed or velocity v is given by
V =
Important Terminology
a) Space mean speed: It represents average speed of vehicles in a certain road length at any time.
Vs =
Where,
Vs = space mean speed (km/h)
d = length of road (m)
n = number of individual vehicle operations
ti = observed travel time (sec) for ith vehicle to travel distance d
The average travel time of all vehicles is obtained from the reciprocal of space mean speed.
b) Time mean speed: represents the speed distribution of vehicles at a point on the roadway and it is the average of instantaneous speed of observed vehicles at the spot
Vt =
Where,
Vt = time mean speed (km/h)
n = number of individual vehicle operations
Vi = observed instantaneous speed of ith vehicle (km/h)
4. Running speed: It is an average speed maintained by a vehicle over a particular stretch of road while the vehicle is in motion.
5. Overall speed or Travel speed: It is an effective speed with which a vehicle traverses a particular route between two terminals.
Speed Study
There are two types of speed studies carried out:
a) Spot Speed Study
b) Speed & Delay Study
Spot Speed Study
Uses:
Factors affecting spot speeds:
Instruments used for finding spot speeds
Presentation of spot speed data
a) Average speed of vehicles
b) Cumulative speed of vehicles
Q6) Explain how the speed and delay studies are carried out.
A6)
Speed & Delay Study
Uses
Types of delays
Methods for carrying out speed & delay study
q =
Where,
q = flow of vehicles in one direction of the stream (volume per min)
na = average number of vehicles counted in the direction of the stream when the test vehicle travels in the opposite direction
ny = average number of vehicles overtaking the test vehicle when the test vehicle travels in the same direction
ta = average journey time when the test vehicle travels in the opposite direction as the stream (mins)
tw = average journey time when the test vehicle travels in the same direction as the stream (mins)
2. License Plate or Vehicle Number Method
3. Interview Technique
Q7) Explain origin and destination study. What are the uses of O & D studies.
A7)
Origin & Destination Study
Uses
Methods of O&D Study
2. License Plate Method
3. Return Post-card Method
4. Tag on car Method
5. Home Interview Method
6. Work Spot Interview Method
Q8) Explain traffic capacity, basic capacity, possible capacity and practical capacity.
A8)
Traffic Capacity (veh/hr/lane): - It is the ability of a roadway to accommodate traffic volume. It depends on the prevailing roadway and traffic conditions.
Traffic volume ≤ Traffic Capacity
Traffic volume represents actual rate of flow and it responds to variation in traffic demand, whereas traffic capacity represents maximum rate of flow possible on the road for given level of service.
Types of traffic capacity
Factors affecting Practical Capacity
Determination of Theoretical Maximum Capacity
C =
Where,
C = Basic capacity of single lane (veh/hr)
V = Speed (km/h)
S = Average centre to centre spacing of vehicles = Sg + L (m)
Sg = Minimum space gap = 0.278Vt (m)
L = Average length of vehicle (m)
t = Reaction Time (sec) – Assumed value is 0.7 sec
Furthermore, the space gap allowed by the driver of a following vehicle depends on several factors such as:
Q9) What are the various types of traffic islands used. Explain the used of each.
A9)
Traffic Island is physical structure or a painted object found on roads and roadside. Traffic island serves various purposes depending on it type. It is usually seen as a raised area along the road for a better & orderly flow of traffic or act as a stopping/ resting area for pedestrians. In few countries traffic islands are also referred to as channelizers as these “channelize” the traffic plying on the road. These may be raised structures made up of concrete or a physical structure in form of boards, barricades, traffic cones etc. The terminology differs as traffic islands are also known as painted islands, ghost islands, roundabouts, refuge islands, rotary etc.
Geometrical Features of Traffic Islands
Since traffic island is a common name given to various road structure and markings, these can be grouped based on the use, location, size, construction type and material etc.
Divisional Islands: To separate opposing flow of traffic on a highway with four or more lanes, hereby preventing head-on collisions.
Channelizing Islands: To guide the flow of traffic into a proper channel through the intersection area.
Pedestrian loading Islands: Provided at bus stops etc for the protection of pedestrians.
Rotary: Channelize the flow of heavy traffic at an intersection.
Q10) What is traffic rotary? What is its advantage and disadvantages in particular reference to traffic conditions in India?
A10)
General
Rotary intersections or round about are special form of at-grade intersections laid out for the movement of traffic in one direction around a central traffic island. Essentially all the major conflicts at an intersection namely the collision between through and right-turn movements are converted into milder conflicts namely merging and diverging. The vehicles entering the rotary are gently forced to move in a clockwise direction in orderly fashion. They then weave out of the rotary to the desired direction.
Advantages and disadvantagesThe key advantages of a rotary intersection are listed below:
Although rotaries offer some distinct advantages, there are few specific limitations for rotaries which are listed below.
Because of the above limitation, rotaries are not suitable for every location. There are few guidelines that help in deciding the suitability of a rotary. They are listed below.
Traffic operations in a rotary
As noted earlier, the traffic operations at a rotary are three; diverging, merging and weaving. All the other conflicts are converted into these three less severe conflicts.