undefined | unit 3 traffic engineering

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

Average Annual Daily Traffic (AADT): The average 24-hour traffic volume at a given location over a full 365-day year, i.e. the total number of vehicles passing the site in a year divided by 365.

Average Annual Weekday Traffic (AAWT): The average 24-hour traffic volume occurring on weekdays over a full year. It is computed by dividing the total weekday traffic volume for the year by 260.

Average Daily Traffic (ADT): An average 24-hour traffic volume at a given location for some period of time less than a year. It may be measured for six months, a season, a month, a week, or as little as two days. An ADT is a valid number only for the period over which it was measured.

Average Weekday Traffic (AWT): An average 24-hour traffic volume occurring on weekdays for some period of time less than one year, such as for a month or a season

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

Travel time: It is an inverse of speed and a simple measure of how well a road network is operating.

Spot speed: It is an instantaneous speed of a vehicle at a specified section or location.

Average speed: It is an average of spot speeds of all vehicles passing a given point on the highway.

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:

In planning traffic control and in traffic regulations

In geometric design

In accident studies

To study traffic capacity

Factors affecting spot speeds:

Physical features of the road like pavement width, curve, sight distance, gradient and roadside developments

Environmental conditions

Enforcement

Traffic conditions

Driver, vehicle and motive of travel

Instruments used for finding spot speeds

Enoscope or Mirror Box Method

Radar speedometer

Graphic recorder

Electronic meter

Photo-electric meter

Presentation of spot speed data

a) Average speed of vehicles

frequency distribution tables are prepared by arranging the data

tables give general information of the speeds distribution pattern

b) Cumulative speed of vehicles

85th percentile speed is the speed at or below which 85% of the vehicles are passing the point on the highway. 15% of the vehicles exceed the speed at that spot. 85th percentile speed is also known as safe speed limit.

98th percentile speed is taken for the purpose of highway geometric design.

15th percentile speed represents the lower speed limit if it is desired to prohibit slow moving vehicles to decrease delay and congestion.

Q6) Explain how the speed and delay studies are carried out.

A6)

Speed & Delay Study

Uses

Give the running speeds, overall speeds, fluctuations in speeds and delay between two stations of a road spaced far apart

Give information such as amount, location, duration, frequency and causes of delay in the traffic stream

Help to find the travel time and in benefit cost analysis

Types of delays

Fixed delay: occurs primarily at intersections due to traffic signals and at level crossings

Operational delay: caused by interference of traffic movements such as turning vehicles, parking & un parking vehicles etc and by internal friction in traffic stream due to high traffic volume, insufficient capacity etc

Methods for carrying out speed & delay study

Floating Car Method

A test vehicle is driven over a given course of travel at approximately the average speed of the stream.

Detailed information is obtained concerning all phases of speed and delay including location, duration and causes of delay.

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

This method does not give important details such as causes of delay and duration etc.

3. Interview Technique

Q7) Explain origin and destination study. What are the uses of O & D studies.

A7)

Origin & Destination Study

Uses

To plan the road network and other facilities for vehicular traffic.

To plan the schedule of different modes of transportation for trip demand of commuters.

To judge the adequacy of existing routes and to use in planning new network of roads.

To locate expressways and major routes along the desire lines.

To establish preferential routes for various categories of vehicles.

To locate the intermediate stops of public transport.

Methods of O&D Study

Roadside Interview Method

Quick collection of data, simple method and team can be trained quickly

Main drawback is that delays occur when vehicles are stopped for interview and it may even lead to resentment of the driver and passengers

2. License Plate Method

Easy and quick method when the area under consideration is quite small

Involves a lot of office compilations in tracing the trips through a network of stations

3. Return Post-card Method

Suitable where traffic is heavy

4. Tag on car Method

Suitable where traffic is heavy

Pre-coded card is stuck on the vehicle to give information at the points of entry and exit into an area, thus indicating the time taken to traverse the area

5. Home Interview Method

Additional data including socio-economic details etc may be collected so as to be useful for forecasting traffic & transportation growth

6. Work Spot Interview Method

Involves collection of O&D data at work spots (offices, factories, educational institutions etc) by personal interviews

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.

Level of service A is the best level of service, because there is no restriction to the movement of driver as very less number of vehicle are moving per kms.

Level of service F is the worst level of service in which there is full restriction on the driver’s movement and it leads to conjunction (Jam).

While designing new roads we provide level of service B.

Types of traffic capacity

Basic Capacity or Theoretical Capacity: It is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour under the most nearly ideal roadway and traffic conditions.

Possible Capacity: It is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour under prevailing roadway and traffic conditions.

Practical Capacity or Design Capacity: It is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour without any delay, hazard or restriction under prevailing roadway and traffic conditions.

Factors affecting Practical Capacity

Lane width: As lane width decreases, capacity increases.

Lateral Clearance: Vertical obstructions such as retaining walls or parked vehicles near the lane reduce the effective lane width and thus result in further reduction of capacity.

Width of Shoulders: Narrow shoulders reduce the effective width of traffic lane as vehicles travel towards the center of the pavement.

Commercial Vehicles: Trucks and buses occupy more space and travel at slow speeds.

Alignment: If alignment and geometric design is not up to standards, the capacity will decrease.

Presence of Intersection at Grades: Intersections restrict free flow and thus adversely affects 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:

Speeds of leading & following vehicles

Type & characteristics of the two vehicles

Driver characteristics of the following vehicle

Road geometrics

Environmental factors

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.

Shape and size: The shape & size of traffic island is determined on the basis of function it is meant to serve. Like in case of median its much longer in length than width because its function is to divide the flow of traffic. In case of a channelizer the function of traffic island is to channelize the flow of traffic in a particular direction. These are sometimes elongated like in case of a channelizers. These can also be round like in case or rotator and round about which are used for giving a larger maneuvering space to drivers.

Location: Traffic island can be present on the side of road, across the road or even in mid way along the road. The functional classification of island is the most important determining factor of the location of an island.

Island type: The decision of using a painted, concrete structure, traffic cone etc rests with the transportation department and can be chosen as per the economics and the extent to which these fulfill the requirement. However if the traffic volume is substantial, concrete structure is generally used as it forces the user to follow it, painted lines work where people are law-abiding and the flow of traffic is relatively less.

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 disadvantages

The key advantages of a rotary intersection are listed below:

Traffic flow is regulated to only one direction of movement, thus eliminating severe conflicts between crossing movements.

All the vehicles entering the rotary are gently forced to reduce the speed and continue to move at slower speed. Thus, none of the vehicles need to be stopped, unlike in a signalized intersection.

Because of lower speed of negotiation and elimination of severe conflicts, accidents and their severity are much less in rotaries.

Rotaries are self governing and do not need practically any control by police or traffic signals.

They are ideally suited for moderate traffic, especially with irregular geometry, or intersections with more than three or four approaches.

Although rotaries offer some distinct advantages, there are few specific limitations for rotaries which are listed below.

All the vehicles are forced to slow down and negotiate the intersection. Therefore, the cumulative delay will be much higher than channelized intersection.

Even when there is relatively low traffic, the vehicles are forced to reduce their speed.

Rotaries require large area of relatively flat land making them costly at urban areas.

The vehicles do not usually stop at a rotary. They accelerate and exit the rotary at relatively high speed. Therefore, they are not suitable when there are high pedestrian movements.

Guidelines for the selection

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.

Rotaries are suitable when the traffic entering from all the four approaches are relatively equal.

A total volume of about 3000 vehicles per hour can be considered as the upper limiting case and a volume of 500 vehicles per hour is the lower limit.

A rotary is very beneficial when the proportion of the right-turn traffic is very high; typically if it is more than 30 percent.

Rotaries are suitable when there are more than four approaches or if there is no separate lanes available for right-turn traffic. Rotaries are ideally suited if the intersection geometry is complex.

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.

Diverging: It is a traffic operation when the vehicles moving in one direction is separated into different streams according to their destinations.

Merging: Merging is the opposite of diverging. Merging is referred to as the process of joining the traffic coming from different approaches and going to a common destination into a single stream.

Weaving: Weaving is the combined movement of both merging and diverging movements in the same direction.

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