Traffic - Traffic Engineering and Management - Lecture Notes, Study notes for Engineering Project Management. Alagappa University

Engineering Project Management

Description: Some concept of Traffic Engineering and Management are Non-Intrusive Technologies, Non-Transportation Designers, Parametric Description, Pedestrian Crossing. Main points of this lecture are: Traffic, Fundamental Parameters, Traffic Engineering, Economical Operation, Mutual Relationships, Stream Parameters, Traffic Engineer, Planning, Traffic Stream, Whole Traffic
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Traffic Engineering And Management 1. Fundamental Parameters of Traffic Flow
Chapter 1
Fundamental Parameters of Traffic
Flow
1.1 Overview
Traffic engineering pertains to the analysis of the behavior of traffic and to design the facilities
for a smooth, safe and economical operation of traffic. Traffic flow, like the flow of water,
has several parameters associated with it. The traffic stream parameters provide information
regarding the nature of traffic flow, which helps the analyst in detecting any variation in flow
characteristics. Understanding traffic behavior requires a thorough knowledge of traffic stream
parameters and their mutual relationships. In this chapter the basic concepts of traffic flow is
presented.
1.2 Traffic stream parameters
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
Dr. Tom V. Mathew, IIT Bombay 1 March 31, 2012
Traffic Engineering And Management 1. Fundamental Parameters of Traffic Flow
includes 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.
1.3 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(1.1)
where, vis the speed of the vehicle in m/s, dis distance traveled in m in time tseconds. Speed
of different vehicles will vary with respect to time and space. To represent these 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.
1.3.1 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 endoscope, 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 traveling by all vehicles between a particular pair of frames.
Dr. Tom V. Mathew, IIT Bombay 2 March 31, 2012
Traffic Engineering And Management 1. Fundamental Parameters of Traffic Flow
1.3.2 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 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
1.3.3 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. A
uniformity between journey and running speeds denotes comfortable travel conditions.
1.3.4 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.
1.4 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 qexpressed in vehicles/hour is given by
q=nt
t(1.2)
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Traffic Engineering And Management 1. Fundamental Parameters of Traffic Flow
Flow is expressed in planning and design field taking a day as the measurement of time.
1.4.1 Variations of Volume
The variation of volume with time, i.e. month to month, day to day, hour to hour and within a
hour is also as important as volume calculation. Volume variations can also be observed from
season to season. Volume will be above average in a pleasant motoring month of summer, but
will be more pronounced in rural than in urban area. But this is the most consistent of all the
variations and affects the traffic stream characteristics the least.
Weekdays, Saturdays and Sundays will also face difference in pattern. But comparing day
with day, patterns for routes of a similar nature often show a marked similarity, which is useful
in enabling predictions to be made.
The most significant variation is from hour to hour. The peak hour observed during morn-
ings and evenings of weekdays, which is usually 8 to 10 per cent of total daily flow or 2 to 3
times the average hourly volume. These trips are mainly the work trips, which are relatively
stable with time and more or less constant from day to day.
1.4.2 Types of volume measurements
Since there is considerable variation in the volume of traffic, several types of measurements of
volume are commonly adopted which will average these variations into a single volume count
to be used in many design purposes.
1. 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.
2. 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.
3. 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.
4. 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.
Dr. Tom V. Mathew, IIT Bombay 4 March 31, 2012
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