Traffic Engineering Course Note
Chapter 2. Traffic survey
2.1 Data collection and analysis
Measurement at a Point
Introduction
The data required by a traffic engineer can mainly be observed on field rather than at laboratory.
Now the field studies can be classified into three types depending upon the length of observation:
1. Measurement at a point
2. Measurement over a short section
3. Measurement over a long section
Out of these we will be discussing the first type here. Flow is the main traffic parameter
measured at a point. Flow can be defined as the no of vehicles passing a section per unit time.
Traffic volume studies are mainly carried out to obtain factual data concerning the movement of
vehicles at selected point on the street or highway system.
2.1.1Basic concepts
Types of Volume Measurement
Volume count varies considerably with time. Hence, several types of measurement of volume are
commonly adopted to average these variations. These measurements are described below:
Average Annual Daily Traffic (AADT)
This is given by the total no. of vehicles passing through a section in a year divided by 365.
This can be used for following purposes:
1. Measuring the present demand for service by the street or highway
2. Developing the major or arterial street
3. Evaluating the present traffic flow with respect to the street system
4. Locating areas where new facilities or improvements to existing facilities are needed.
Average Annual Weekday Traffic (AAWT)
This is defined as the average 24-hour traffic volume occurring on weekdays over a full year.
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.
1
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
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.
2.1.2 Type of Counts
Various types of traffic counts are carried out, depending on the anticipated use of the data to be
collected. They include:
Cordon Count
These are made at the perimeter of an enclosed area (CBD, shopping center etc.). Vehicles or
persons entering and leaving the area during a specified time period are counted.
Screen Line Count
These are classified counts taken at all streets intersecting an imaginary line (screen line)
bisecting the area. These counts are used to determine trends, expand urban travel data, traffic
assignment etc.
Pedestrian Count
These are used in evaluating sidewalk and crosswalk needs, justifying pedestrian signals, traffic
signal timings etc.
Intersection Count
These are measured at the intersections and are used in planning turn prohibitions,
designing channelization, computing capacity, analyzing high accident intersections etc.
2.1.3 Counting Techniques
Number of vehicles can be counted either manually or by machine depending upon the duration
of study, accuracy required, location of study area etc.
Manual counting
In its simplest form an observer counts the numbers of vehicles along with its type, passing
through the section for a definite time interval. For light volumes, tally marks on a form are
adequate. Mechanical or electrical counters are used for heavy traffic. Although it is good to take
some manual observations for every counting for checking the instruments, some other specific
uses of manual counts are following:
1. Turning and through movement studies
2. Classification and occupancy studies
3. For analysis of crosswalks, sidewalks, street corner space and other pedestrian facilities
2
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Automatic counting
These can be used to obtain vehicular counts at non-intersection points. Total volume, directional
volume or lane volumes can be obtained depending upon the equipment available.
Permanent Counters
These can be mainly grouped into contact types, pulsed types, radar types. Among the contact
type counters, pneumatic tubes are mostly used. Air pulse actuated by vehicle wheels, pass along
the tube thereby increasing the count. Pulsed types mainly depend upon the interruption of a
beam generated from a station located near the site, which is detected by the receiver. In radar
types, a continuous beam of energy is directed towards the vehicle.
Portable Counters
These are used to obtain temporary or short term counts. Generally these make use of a
transducer unit actuated by energy pulses. Each axle or vehicle passage operates a switch
attached to a counter which is usually set to register one unit for every two axles
2.1.4 Counting Periods
Counting periods vary from short counts at spot points to continuous counts at permanent
stations. Hourly counts are generally significant in all engineering design, while daily and annual
traffic is important in economic calculations, road system classification and investment
programmes.
Some of the more commonly used intervals are:
1. 24-hour counts normally covering any 24-hour period between noon Monday and noon
Friday. If a specific day count is desired, the count should be from midnight to midnight.
2. 16 hour counts usually 5:30 am to 9:30 pm or 6 am to 9 pm.
3. 12 hour counts usually from 7 am to 7 pm
4. Peak Period counting times vary depending upon size of metropolitan area, proximity to major
generators and the type of facility. Commonly used periods are 7 to 9 am and 4 to 6 pm.
2.1.5 Variation of Volume Counts and Peak Hour Factors
Variation of volume counts can be further sub-divided into daily, weekly and seasonal variation.
For studying the daily variation, the flow in each hour has been expressed as percentage of daily
flow. Weekdays, Saturdays and Sundays usually show different patterns. Peak Hour Volume is
very important factor in the design of roads and control of traffic, and is usually 2 - 2.5 times the
3
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
average hourly volume. Apart from this there is one additional feature of this variation: two
dominant peaks (morning and evening peak), especially in urban areas.
Peak hour factors should be applied in most capacity analyses in accordance with the Highway
Capacity Manual, which selected 15 minute flow rates as the basis for most of its procedures.
The peak-hour factor (PHF) is descriptive of trip generation patterns and may apply to an area or
portion of a street and highway system. The PHF is typically calculated from traffic counts. It is
the average volume during the peak 60 minute period Vav60 divided by four times the a
maximum volume during the peak 15 minute’s period V max15 .
Numerical Example
The table below shows the volumetric data observed at an intersection. Calculate the peak hour
volume, peak hour factor (PHF), and the actual (design) flow rate for this approach.
Solution we can locate the hour with the highest volume and the 15 minute interval with the
highest volume. The peak hour volume is just the sum of the volumes of the four 15 minute
4
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
intervals within the peak hour (219). The peak 15 minute volume is 65 in this case. The peak
hour factor (PHF) is found by dividing the peak hour volume by four times the peak 15 minute
volume.
The actual (design) flow rate can be calculated by dividing the peak hour volume by the PHF,
219/0.84 = 260 vehicles/hr, or by multiplying the peak 15 minute volume by four, 4 × 65 = 260
vehicles per hour.
2.1.6 Over a Short Section
The main purpose of this topic is to determine traffic parameter, specially speed. Speed
measurements are most often taken at a point (or a short section) of road way under conditions of
free flow. The intent is to determine the speeds that drivers select, unaffected by the existence of
congestion.
2.1.6.1 Speed Studies
The actual speed of traffic flow over a given route may fluctuated widely, as because at each time
the volume of traffic varies. Accordingly, speeds are generally classified into three main
categories
1. Spot speed this is the instantaneous speed of a vehicle at any specific location.
2. Running speed this is the average speed maintained over a particular course while the vehicle
is in the motion.
3. Journey speed This is the effective speed of the vehicle on a journey between two points and
the distance between two points, and the distance between these points divided by the total time
taken for the vehicle to complete the journey.
2.1.9Measurement along a Length of Road
Overview
This is normally used to obtain variations in speed over a stretch of road. Usually the stretch will
be having a length more than 500 meters. We can also get speed, travel time and delay. Speed
and travel time are the most commonly used indicators of performance for traffic facilities and
networks. Delays are often used to measure the performance of traffic flow at intersections.
5
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
2.1.10 Travel time study
Travel time is the elapsed time it takes for a vehicle to traverse a given segment of a street.
Travel time studies provide the necessary data to determine the average travel time. Combined
with the length of the corridor under study, this data can be used to produce average travel speed.
Since vehicle speed is directly related to travel time and delay, it is also an appropriate measureof-performance to evaluate traffic systems. Travel time may be defined as the total elapsed time
of travel, including stop and delay, necessary for a vehicle to travel from one point to another
point over a specified route under existing traffic condition.
2.1.11Delay studies
Delay is defined as an extra time spent by drivers against their expectation. Delay can have many
forms depending on different locations. A study made to provide information concerning the
amount, cause, location, duration and frequency of delay as well as travel time and similar value.
The time lost by traffic due to traffic friction and traffic control device is called delay.
2.1.12 Types of Delay
1. Congestion delay- Congestion delay is the delay caused by the constricting or slowing down
effect of overloaded intersections, inadequate carriageway widths, parked cars, crowded
pavement and similar factor.
2. Fixed Delay- The delay to which a vehicle is subjected regardless of the amount of traffic
volume and interference present.
3. Operational Delay-The delay caused by interference from other component of the traffic
stream. Examples include time lost while waiting for a gap in a conflicting traffic stream, or
resulting from congestion, parking maneuvers, pedestrians, and turning movement.
4. Stopped Delay- The time a vehicle is not moving.
5. Travel Time Delay- The difference between the actual time required to traverse a section of
street or highway and the time corresponding to the average speed of trffic under uncongested
condition.
6. Approach Delay -Travel time delay encountered to an approach to an intersection.
6
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
2.1.13 Purpose of travel time and Delay Studies
1. The purpose of a Travel Time and Delay Study is to evaluate the quality of traffic movement
along a route and determine the locations, types, and extent of traffic delays by using a moving
test vehicle.
2. This study method can be used to compare operational conditions before and after roadway or
intersection improvements have been made.
3. The Travel Time and Delay Study can also be used by planners to monitor level of service for
local government comprehensive plans.
2.1.13.1 Method for obtaining travel time and delay study
1. Floating Car Method: Floating car data are positions of vehicles traversing city streets
throughout the day. In this method the driver tries to float in the traffic stream passing as many
vehicles as pass the test car. If the test vehicle overtakes as many vehicles as the test vehicle is
passed by, the test vehicles should, with sufficient number of runs, approach the median speed of
the traffic movement on the route.
2. Average Speed Method: In this method the driver is instructed to travel at a speed that is
judge to the representative of the speed of all traffic at the time.
3. Moving-vehicle method: In this method, the observer moves in the traffic stream and makes a
round trip on a test section. The observer starts at section, drives the car in a particular direction
say eastward to another section, turns the vehicle around drives in the opposite direction say
westward toward the previous section again.
4. Maximum-car method: In this procedure, the driver is asked to drive as fast as is safely
practical in the trffic stream without ever exceeding the design speed of the facility.
5. Elevated Observer method: In urban areas, it is sometime possible to station observers in
high buildings or other elevated points from which a considerable length of route may be
observed. These investigator select vehicle at random and record time, location, and causes of
delay.
2.1.14 Spot Speed Study
Spot Speed is the average speed of vehicles passing a point, or the time mean speed.
Spot Speed studies are conducted to estimate the distribution of speeds of vehicles in a stream of
traffic at a particular location on a highway. This is carried out by recording the speeds of a
sample of vehicles at a specified location.
7
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Application of Spot Speeds
1. Speed Limit Studies
2. Establishing Speed Trends
3. Specific Design Applications
4. Specific Control Applications
5. Investigation of High Accident Locations
Time and duration
The time of day for conducting a speed study depends on the purpose of the study.
In general, when the purpose of the study is to establish posted speed limits, to observe speed
trends, or to collect basic data, it is recommended that the study be conducted when traffic is free
flowing, usually during off-peak hours.
Typically, the duration is at least 1 hour and the sample size is at least 30 vehicles.
Data Presentation
The speed data can be presented by:
1. Frequency Distribution Table, and
2. Frequency and Cumulative Frequency Distribution Curves.
1. Frequency Distribution Table

The individual speeds of vehicles collected from the field are used to prepare the
frequency distribution table.

The frequency distribution table shows the total number of vehicles observed in each
speed group.

Speed groups of more than 5 mph are not used.
8
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
9
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
2. Frequency and Cumulative Frequency
Distribution Curves
Curves are prepared from the Frequency Distribution Table. Once the points are plotted,
they are connected by a smooth curve. They are usually plotted one above the other,
using the same horizontal axis for speed.
The frequency distribution curve plots points which represent the middle speed of each
speed group versus the % frequency in the speed group.
Since the cumulative % frequency is defined as the percentage of vehicles traveling at or
below a given speed, the cumulative frequency distribution curve plots the upper limit of
the speed group (NOT the middle speed).
10
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
 Precision and Confidence Interval
The confidence interval for the true mean is
For the example problem, standard deviation of the sample is 4.94 mph, sample size is 283,
and the sample mean speed is 48.1 mph.
 The 95% confidence interval for the true mean speed is 48.1± 1.96(0.294) mph or
from 47.52 mph to 48.68 mph.
 Therefore, we can be 95% confident that the true mean speed would be between
47.52 mph and 48.68 mph.
2.2 Forecasting Future Traffic Flows
2.2.1 Basic principles of traffic demand analysis
If transport planners wish to modify a highway network either by constructing a new roadway or
by instituting a programme of traffic management improvements, any justification for their
proposal will require them to be able to formulate some forecast of future traffic volumes along
the critical links.
Particularly in the case of the construction of a new roadway, knowledge of the traffic volumes
along a given link enables the equivalent number of standard axle loadings over its lifespan to be
estimated, leading directly to the design of an allowable pavement thickness, and provides the
basis for an appropriate geometric design for the road, leading to the selection of a sufficient
11
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
number of standard width lanes in each direction to provide the desired level of service to the
driver.
The prediction of highway demand requires a unit of measurement for travel behaviour to be
defined. This unit is termed a trip and involves movement from a single origin to a single
destination. The parameters utilized to detail the nature and extent of a given trip are as follows:

Purpose

Time of departure and arrival

Mode employed

Distance of origin from destination

Route travelled.
2.2.1.3 Trip generation
Trip generation models provide a measure of the rate at which trips both in and out of the zone in
question are made. They predict the total number of trips produced by and attracted to its zone.
Centers of residential development, where people live, generally produce trips. The more dense
the development and the greater the average household income is within a given zone, the more
trips will be produced by it. Centers of economic activity, where people work, are the end point
of these trips. The more office, factory and shopping space existing within the zone, the more
journeys will terminate within it.
It is an innately difficult and complex task to predict exactly when a trip will occur. This
complexity arises from the different types of trips that can be undertaken by a car user during the
course of the day (work, shopping, leisure, etc.).
The process of stratification attempts to simplify the process of predicting the number and type
of trips made by a given zone. Trips are often stratified by purpose, be it work, shopping or
relaxation. Different types of trips have different characteristics that result in them being more
likely to occur at different times of the day. The peak time for the journey to work is generally in
the early morning, while shopping trips are most likely during the early evening. Stratification by
time, termed temporal aggregation, can also be used, where trip generation models predict the
number of trips per unit timeframe during any given day.
Within the context of an urban transportation study, three major variables govern the rate at
which trips are made from each zone within the study area:
Distance of zone from the central business district/city center area
12
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Socio-economic characteristics of the zone population (per capita income, cars available
per household)
Intensity of land use (housing units per hectare, employees per square meter of office
space).
The relationships between trips generated and the relevant variables are expressed as
mathematical equations, generally in a linear form. For example, the model could take the
following form:
Where
Tij = number of vehicle trips per time period for trip type i (work, non-work) made by household j
Z = characteristic value n for household j, based on factors such as the household income level
and number of cars available within it.
∝=regression coefficient estimated from travel survey data relating to n
A typical equation obtained for a transportation study might be:
where
T = total number of trips per household per 24 hours
Z = family size
Z1= total income of household
Z2= cars per household
Z4= housing density
13
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
For a given trip generation equation, the coefficients can be assumed to remain constant over
time for a given specified geographical location with uniform demographic and socio-economic
factors.
For example, the more people within a household and the more cars available to them, the more
trips they will make; say we define 15 subgroups in terms of two characteristics – numbers
within the household and number of cars available and we estimate the number of trips each
subgroup is likely to make during the course of the day. An example of category analysis figures
is given in Table 2.1.
14
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
2.2.1.4 Trip distribution
2.2.1.4. a Introduction
The previous model determined the number of trips produced by and attracted to each zone
within the study area under scrutiny. For the trips produced by the zone in question, the trip
distribution model determines the individual zones where each of these will end. For the trips
ending within the zone under examination, the individual zone within which each trip originated
is determined. The model thus predicts zone-to-zone trip interchanges. The process connects two
known sets of trip ends but does not specify the precise route of the trip or the mode of travel
used. These are determined in the two last phases of the modeling process.
The end product of this phase is the formation of a trip matrix between origins and destinations,
termed an origin-destination matrix. Its layout is illustrated in Table 2.3.
There are several types of trip distribution models, including the gravity model and the Furness
method.
Table 2.3 Origin destination Matrix (e.g. T14 = number of trips originating in
zone 1 and ending in zone 4)
2.2.1.4 .b The gravity model
The gravity model is the most popular of all the trip distribution models. It allows the effect of
differing physical planning strategies, travel costs and transportation systems to be taken into
account.
Within it, existing data is analyzed in order to obtain a relationship between trip volumes and the
generation and attraction of trips along with impedance factors such as the cost of travel.
15
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
The name is derived from its similarity to the law of gravitation put forward by Newton where
trip interchange between zones is directly proportional to the attractiveness of the zones to trips,
and inversely proportional to some function of the spatial separation of the zones.
The gravity model exists in two forms:
Where
T = trips from zone i to zone j
Aj = trip attractions in zone j
Pi= trip productions in zone i
Fij = impedance of travel from zone i to zone j
2.3 Measurement of traffic parameters
2.3.1 General
The traffic stream includes a combination of driver and vehicle behavior. The driver or human
behavior being non-uniform, traffic stream is also non-uniform in nature. It is influenced not
only by the individual characteristics of both vehicle and human but also by the way a group of
such units interacts with each other. Thus a flow of traffic through a street of defined
characteristics will vary both by location and time corresponding to the changes in the human
behavior.
The traffic engineer, but for the purpose of planning and design, assumes that these changes are
within certain ranges which can be predicted. For example, if the maximum permissible speed of
a highway is 60 kmph, the whole traffic stream can be assumed to move on an average speed of
40 kmph rather than 100 or 20 kmph.
Thus the traffic stream itself is having some parameters on which the characteristics can be
predicted. The parameters can be mainly classified as: measurements of quantity, which includes
16
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
density and flow of traffic and measurements of quality which includes speed. The traffic stream
parameters can be macroscopic which characterizes the traffic as a whole or microscopic which
studies the behavior of individual vehicle in the stream with respect to each other.
As far as the macroscopic characteristics are concerned, they can be grouped as measurement of
quantity or quality as described above, i.e. flow, density, and speed. While the microscopic
characteristics include the measures of separation, i.e. the headway or separation between
vehicles which can be either time or space headway. The fundamental stream characteristics are
speed, flow, and density and are discussed below.
2.3.2 Speed
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 = 𝑑/𝑡
Where, v is the speed of the vehicle in m/s, d is distance traveled in m in time t seconds. Speed of
different vehicles will vary with respect to time and space. To represent this variation, several
types of speed can be defined. Important among them are spot speed, running speed, journey
speed, time mean speed and space mean speed. These are discussed below.
2.3.2.1 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.
Location and size of signs, design of signals, safe speed, and speed zone determination, require
the spot speed data. Accident analysis, road maintenance, and congestion are the modern fields
of traffic engineer, which uses spot speed data as the basic input. Spot speed can be measured
using an enoscope, 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.
2.3.2.2 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. i.e. this speed doesn’t consider the time during which the vehicle is brought to a stop, or
17
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
has to wait till it has a clear road ahead. The running speed will always be more than or equal to
the journey speed, as delays are not considered in calculating the running speed.
2.3.2.3 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.
Uniformity between journey and running speeds denotes comfortable travel conditions.
2.3.2.4 Time mean speed and space mean speed
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
traveling 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.
2.3.3 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=
𝑛𝑡
𝑡
Flow is expressed in planning and design field taking a day as the measurement of time.
2.3.3.2 Density
Density is defined as the number of vehicles occupying a given length of highway or lane and is
generally expressed as vehicles per km. One can photograph a length of road x, count the number
of vehicles, nx, in one lane of the road at that point of time and derive the density k as,
𝑘 = 𝑛𝑥/𝑥
18
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
The density is the number of vehicles between the point A and B divided by the distance
between A and B. Density is also equally important as flow but from a different angle as it is the
measure most directly related to traffic demand. Again it measures the proximity of vehicles in
the stream which in turn affects the freedom to maneuver and comfortable driving.
2.3.3.3Time headway
The microscopic character related to volume is the time headway or simply headway. Time
headway is defined as the time difference between any two successive vehicles when they cross
a given point. Practically, it involves the measurement of time between the passage of one rear
bumper and the next past a given point. If all headways h in time period, t, over which flow has
been measured are added then,
But the flow is defined as the number of vehicles nt measured in time interval t, that is,
Where, hav is the average headway. Thus average headway is the inverse of flow. Time headway
is often referred to as simply the headway.
2.3.3.4 Distance headway
Another related parameter is the distance headway. It is defined as the distance between
corresponding points of two successive vehicles at any given time. It involves the measurement
from a photograph, the distance from rear bumper of lead vehicle to rear bumper of following
vehicle at a point of time. If all the space headways in distance x over which the density has been
measured are added,
19
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Where, sav is average distance headway. The average distance headway is the inverse of density
and is sometimes called as spacing.
2.3.3.5 Travel time
Travel time is defined as the time taken to complete a journey. As the speed increases, travel
time required to reach the destination also decreases and vice versa. Thus travel time is inversely
proportional to the speed.
However, in practice, the speed of a vehicle fluctuates over time and the travel time represents an
average measure.
2.3.4 Generation of traffic congestion
The change in transport system causes a change in transport behavior and locational pattern of
the system. The change in household characteristics, transport behavior, locational pattern, and
other growth effects result in the growth of traffic. But the change or improvement in road
capacity is only as the result of change in the transportation system and hence finally a situation
arises where the traffic demand is greater than the capacity of the roadway. This situation is
called traffic congestion.
2.3.4.1 Effects of congestion
Congestion has a large number of ill effects on drivers, environment, health and the economy in
the following ways.
• Drivers who encounter unexpected traffic may be late for work and other appointments causing
a loss in productivity and their valuable time.
• Since congestion leads to increase in travel time i.e., vehicles are made to travel for more time
than required which consumes large amount of fuel there by causing fuel loss and economic loss
to the drivers.
• One of the most harmful effects of traffic congestion is its impact on the environment.
Despite the growing number of vehicles, a car stopped in traffic still produces a large volume of
harmful carbon emissions. Increase in pollutants (because of both the additional fuel burned and
more toxic gases produced while internal combustion engines are in idle or in stop-and-go
traffic)
• Drivers who become impatient may be more likely to drive aggressively and dangerously and
leads to high potential for traffic accidents
20
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
• Negative impact on people’s psychological state, which may affect productivity at work and
personal relationships
• Slow and inefficient emergency response and delivery services
• Decrease in road surface lifetime: When a vehicle moves over the surface, the areas of contact
(where the vehicles’ tyres touch the road) are deflected downwards under the weight of the
vehicle and as the vehicle moves forward, the deflection corrects itself to its original position.
• Vehicle maintenance costs; ’Wear and tear’ on mechanical components of vehicles such as the
clutch and brakes is also considerably increased under stop-start driving conditions and hence
increasing the vehicle maintenance costs.
• One beneficial effect of traffic congestion is its ability to encourage drivers to consider other
transportation options like a subway, light rail or bus service. These options reduce traffic on the
roads, thereby reducing congestion and environmental pollution.
The summation of all these effects yields a considerable loss for the society and the economy of
an urban area.
2.3.4.2 Traffic congestion
A system is said to be congested when the demand exceeds the capacity of the section. Traffic
congestion can be defined in the following two ways:
1. Congestion is the travel time or delay in excess of that normally incurred under light or free
flow traffic condition.
2. Unacceptable congestion is travel time or delay in excess of agreed norm which may vary by
type of transport facility, travel mode, geographical location, and time of the day.
Traffic congestion may be of two types:
1. Recurrent Congestion: Recurrent congestion generally occurs at the same place, at the same
time every weekday or weekend day. This is generally the consequence of factors that act
regularly or periodically on the transportation system such as daily commuting or weekend trips.
Recurrent congestion is predictable and typically occurs during peak hours. It displays a large
degree of randomness in terms of duration and severity.
2. Non-Recurrent congestion: Non-Recurrent congestion is the effect of unexpected, unplanned large events( road woks, accidents, special events and so on) that affect transportation
system more or less randomly and as such, cannot be easily predicted.
21
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
2.3.4.3 Measurement of congestion
Need and uses of congestion measurement
Congestion has to be measured or quantified in order to suggest suitable counter measures and
their evaluation. Congestion information can be used in a variety of policy, planning and
operational situations. It may be used by public agencies in assessing facility or system
adequacy, identifying problems, calibrating models, developing and assessing improvements,
formulating programs policies and priorities. It may be used by private sector in making
locational or investment decisions. It may be used by general public and media in assessing
traveler’s satisfaction.
2.3.4.4 System performance measurement
Performance measure of a congested roadway can be done using the following four components:
1. Duration,
2. Extent,
3. Intensity, and
4. Reliability.
Duration
Duration of congestion is the amount of time the congestion affects the travel system. The peak
hour has now extended to peak period in many corridors. Measures that can quantify congestion
include:
• Amount of time during the day that the travel rate indicates congested travel on a system
element or entire system.
• Amount of time during the day that traffic density measurement techniques (detectors, aerial
surveillance, etc.) indicate congested travel.
Duration of congestion is the sum of length of each analysis sub period for which the demand
exceeds capacity. This component measures the performance of a particular road in handling
traffic efficiently i.e., with the increase in the duration of congestion, poorer will be the
performance of the transportation system. The maximum duration on any link indicates the
amount of time before congestion is completely cleared from the corridor. Duration of
congestion can be computed for a corridor using the following equation: For corridor analysis,
22
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Where, H is the duration of congestion (hours), N is the number of analysis sub periods for
which v/c > 1, and T is the duration of analysis sub-period (hours). For area wide analysis,
where, Hi is the duration of congestion for link i (hours), T is the duration of analysis period
(hours), r is the ratio of peak demand to peak demand rate, vi is the vehicle demand on link I
(veh/hr), and ci is the capacity of link i (veh/hr).
Queue density default values
Extent
Extent of congestion is described by estimating the number of people or vehicles affected by
congestion and by the geographic distribution of congestion. These measures include:
1. Number or percentage of trips affected by congestion.
2. Number or percentage of person or vehicle meters affected by congestion.
3. Percentage of the system affected by congestion.
Performance measures of extent of congestion can be computed from sum of length of queuing
on each segment. Segments in which queue overflows the capacity are also identified. This is
useful for ramp metering analysis. To compute queue length, average density of vehicles in a
queue need to be known. Queue length can be found out using the equation:
23
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Where; QLi is the queue length (meter), v is the segment demand (veh/hour), c is the segment
capacity (veh/hour), N is the number of lanes, ds is the storage density (veh/meter/lane), and
T is the duration of analysis period (hour). If v < c, Qi=0 the equation for queue length is similar
for both corridor and area-wide analysis.
Numerical example
Consider a road segment of 6 lanes with a capacity of 2400 veh/hr/lane. It is observed that the
storage density is 75 veh/meter and the segment demand is found to be 2800 veh/hr/lane.
Given that the duration of analysis sub period is 2 hrs calculate the queue length that is formed
due to congestion.
Solution The queue length of a particular road segment is given by,
It is given that Number of lanes, N=6, Duration of analysis sub period, T= 2 hrs, Segment
Capacity=c=2400 veh/hr/lane, Segment Demand=v=2800 veh/hr/lane, Storage Density=ds=75
veh/ meter. Now, the queue length can be calculated by using the above formula as follows:
= 10.667mts Therefore,
the extent of congestion in terms of queue length is 10.667mts.
Intensity
Intensity of congestion marks the severity of congestion. It is used to differentiate between levels
of congestion on transport system and to define total amount of congestion. It is measured in
terms of:
• Delay in person hours or vehicle hours;
• Average speed of roadway, corridor, or network;
• Delay per capita or per vehicle travelling in the corridor, or per person or per vehicle affected
by congestion;
• Relative delay rate (relative rate of time lost for vehicles);
Intensity in terms of delay is given by,
24
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Where, DPH is the person hours of delay, TPH is the person hours of travel under actual
conditions, and T0 PH is the person hours of travel under free flow conditions. The TPH is given
by:
Where, OAV is the average vehicle occupancy, v is the vehicle demand (veh), l is the length of
link (km), and S is the mean speed of link (km/hr). The TPH is given by:
Where, OAV is the average vehicle occupancy, v is the vehicle demand (veh), l is the length of
link (km), and S0 is the free flow speed on the link (km/hr).
Numerical example
On a 2.8 km long link of road, it was found that the demand is 1000 Vehicles/hour mean speed
of the link is 12 km/hr, and the free flow speed is 27 km/hr. assuming that the average vehicle
occupancy is 1.2 person/vehicle, calculate the congestion intensity in terms of total person hours
of delay.
Solution: Given data: Length of the link=l=2.8 km, Vehicle demand=v=1000 veh, Mean
Speed of the link=S=12 km/hr, Free flow speed on the link= so=27 km/hr, and Average Vehicle
Occupancy= OAv=1.2 person/veh. Person hours of delay are given as:
Person hours of travel under actual conditions,
25
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Therefore, person hours of delay can be calculated as follows,,
DPH = = 280 − 124.4
= 155.6 person hours
= 156 person hours (approx).
Hence, the intensity of congestion is determined in terms of person hours of delay as 156 person
hours.
2.3.5 Parking Studies
Overview
Parking is one of the major problems that is created by the increasing road traffic. It is an impact
of transport development. The availability of less space in urban areas has increased the demand
for parking space especially in areas like Central business district. This affects the mode choice
also. This has a great economic impact.
Parking system
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. The standard dimensions of a car is taken as 5× 2.5 meters and that
for a truck is 3.75× 7.5 meters.
1. 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 most
safest parking from the accident perspective. However, it consumes the maximum curb length
and therefore only a minimum number of vehicles can be parked for a given kerb length. This
method of parking produces least obstruction to the on-going traffic on the road since least road
width is used. Parallel parking of cars is shown in figure below.
26
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
The length available to park N number of vehicles, L = N/5.9
2. 30◦ parking: In thirty degree parking, the vehicles are parked at 30◦ with respect to the road
alignment. In this case, more vehicles can be parked compared to parallel parking.
Illustration of parallel parking
Illustration of 30◦ parking
Also there is better maneuverability. Delay caused to the traffic is also minimum in this
type of parking.
3. 45◦ parking: As the angle of parking increases, more number of vehicles can be parked.
4. 60◦ parking: The vehicles are parked at 60◦ to the direction of road. More number of vehicles
can be accommodated in this parking type.
5. Right angle parking: In right angle parking or 90◦ parking, the vehicles are parked
perpendicular to the direction of the road. Although it consumes maximum width kerb.
27
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Illustration of 45◦ parking
Illustration of 60◦ parking.
length required is very little. In this type of parking, the vehicles need complex maneuvering and
this may cause severe accidents. This arrangement causes obstruction to the road traffic
particularly if the road width is less. However, it can accommodate maximum number of
vehicles for a given kerb length. Length available for parking N number of vehicles is L = 2.5N.
Off street parking
In many urban centers, some areas are exclusively allotted for parking which will be at some
distance away from the main stream of traffic. Such a parking is referred to as off-street parking.
They may be operated by either public agencies or private firms. A typical layout of an off-street
parking is shown in figure below.
28
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Illustration of 90◦ parking
Illustration of off-street parking
Parking requirements
There are some minimum parking requirements for different types of building. For residential
plot area less than 300 sq.m require only community parking space. For residential plot area
from 500 to 1000 sq.m, minimum one-fourth of the open area should be reserved for parking.
Offices may require at least one space for every 70 sq.m as parking area. One parking space
is enough for 10 seats in a restaurant where as theatres and cinema halls need to keep only 1
parking space for 20 seats. Thus, the parking requirements are different for different land use
zones.
29
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Ill effects of parking
Parking has some ill-effects like congestion, accidents, pollution, obstruction to fire-fighting
operations etc.
1. Congestion: Parking takes considerable street space leading to the lowering of the road
capacity. Hence, speed will be reduced, journey time and delay will also subsequently increase.
The operational cost of the vehicle increases leading to great economical loss to the community.
2. Accidents: Careless maneuvering of parking and unparking leads to accidents which are
referred to as parking accidents. Common type of parking accidents occur while driving out a car
from the parking area, careless opening of the doors of parked cars, and while bringing in the
vehicle to the parking lot for parking.
3. Environmental pollution: They also cause pollution to the environment because stopping and
starting of vehicles while parking and unparking results in noise and fumes. They also affect the
aesthetic beauty of the buildings because cars parked at every available space creates a feeling
that building rises from a plinth of cars.
4. Obstruction to firefighting operations: Parked vehicles may obstruct the movement of
firefighting vehicles. Sometimes they block access to hydrants and access to buildings.
Parking statistics
Before taking any measures for the betterment of conditions, data regarding availability of
parking space, extent of its usage and parking demand is essential.
Parking surveys are intended to provide all these information.
Since the duration of parking varies with different vehicles, several statistics are used to access
the parking need. The following parking statistics are normally important.
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.
2. Parking volume: Parking volume is the total number of vehicles parked at a given duration of
time. This does not account for repetition of vehicles. The actual volume of vehicles entered in
the area is recorded.
3. Parking load: Parking load gives the area under the accumulation curve. It can also be
obtained by simply multiplying the number of vehicles occupying the parking area at each time
interval with the time interval. It is expressed as vehicle hours.
30
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
4. Average parking duration: It is the ratio of total vehicle hours to the number of vehicles
parked.
5. Parking turnover: It is the ratio of number of vehicles parked in duration to the number of
parking bays available. This can be expressed as number of vehicles per bay per time duration.
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:
Numerical Example
To illustrate the various measures, consider a small example in figure 41:7, which shows the
duration for which each of the bays are occupied(shaded portion). Now the accumulation graph
can be plotted by simply noting the number of bays occupied at time interval of 15, 30, 45 etc.
minutes is shown in the figure. The various measures are calculated as shown below: Parking
31
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Parking bays and accumulation curve
Parking surveys
Parking surveys are conducted to collect the above said parking statistics. The most common
parking surveys conducted is in-out survey, fixed period sampling and license plate method of
survey.
In-out survey
In this survey, the occupancy count in the selected parking lot is taken at the beginning. Then the
number of vehicles that enter the parking lot for a particular time interval is counted. The number
of vehicles that leave the parking lot is also taken. The final occupancy in the parking lot is also
taken. Here the labor required is very less. Only one person may be enough. But we wont get any
data regarding the time duration for which a particular vehicle used that parking lot. Parking
duration and turnover is not obtained. Hence we cannot estimate the parking fare from this
survey. For quick survey purposes, a fixed period sampling can also be done.
This is almost similar to in-out survey. All vehicles are counted at the beginning of the survey.
Then after a fixed time interval that may vary between 15 minutes to i hour, the count is again
taken. Here there are chances of missing the number of vehicles that were parked for a short
duration.
32
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
Numerical Example
From an in-out survey conducted for a parking area consisting of 40 bays, the initial count was
found to be 25. Table gives the result of the survey. The number of vehicles coming in and out of
the parking lot for a time interval of 5 minutes is as shown in the table below. Find the
accumulation, total parking load, average occupancy and efficiency of the parking lot.
In-out survey data
• Accumulation can be found out as initial count plus number of vehicles that entered the parking
lot till that time minus the number of vehicles that just exited for that particular
For the first time interval of 5 minutes, accumulation can be found out as
time interval.
25+3-2 = 26. It
is being tabulated in column 4.
• Occupancy or parking index is given by equation For the first time interval of five minutes,
Parking index = 26/40 × 100 = 65%. The occupancy for the remaining time slot is similarly
calculated and is tabulated in column 5. Average occupancy is the average of the occupancy
values for each time interval. Thus it is the average of all values given in column 5 and the value
is 80.63%.
33
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute
Traffic Engineering Course Note
• Parking load is tabulated in column 6. It is obtained by multiplying accumulation with the time
interval. For the first time interval, parking load = 26 × 5 = 130 vehicle minutes.
• Total parking load is the summation of all the values in column 5 which is equal to 1935
vehicle minutes or 32.25 vehicle hours
Solution The solution is shown in table
In-out parking survey solution
34
Prepared by Aklilu Tefera Federal Technical and Vocational Education and Training Institute