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Traffic Engineering Studies
(Volume Studies)
Chapter 4
 Traffic volume studies are conducted to collect data on
the number of vehicles and/or pedestrians that pass a
point on the highway during a specified time period.
 Time period varies from 15 minutes to one year
depending on the use of data.
 The data may be put into subclasses:
 Directional movement.
 Occupancy rates.
 Vehicle classification.
 Pedestrian age.
 Volume studies are usually conducted when certain
volume characteristics are needed, some of which
Volume Characteristics
1- Average Annual Daily Traffic (AADT):
 Is the average of 24-hr counts collected every day in
the year.
 AADT used in several traffic and transportation
analysis that includes:
 Estimate of highway user revenues.
 Computation of accident rates in terms of accident
per 100 million vehicle-miles.
 Establishment of traffic volume trends.
 Development of freeways and major arterial street
 Development and improvement of maintenance
Volume Characteristics
2- Average Daily Traffic (ADT)
 Is the average of 24-hr counts
collected over a number of days (> 1
and <1 year)
 ADT may be used for:
 Planning of highway activities.
 Measurements of current demands.
 Evaluation of existing traffic flow.
Volume Characteristics
4- Peak Hour Volume (PHV)
 Is the maximum number of vehicle that pass a point
during a period of 60 consecutive minutes.
 PHVs are used for:
 Functional classification of highways.
 Design of the geometric characteristics of highways
(e.g. number of lanes, intersection signalization and
 Capacity analysis.
 Development of parking regulations.
 Development of traffic operation programs (e.g. oneway street system or traffic routing).
Volume Characteristics
4- Vehicle Classification (VC)
 Records volume with respect to the type of
vehicles (e.g. passenger cars, two-axle
trucks, …)
 VCs are used for:
 Design of geometric characteristics (turning radii
requirements, maximum grades, lane width,…).
 Capacity analysis (ESAL).
 Adjustment of traffic counts obtained by
 Structural design of highway pavements and
Volume Characteristics
5- Vehicle Miles of travel (VMT)
 Is a measure of travel a long a section of
 VMT = (ADT or average week day volume)
x (length of roadway in miles)
 VMTs are used for:
 Base for allocating resources for maintenance
and improvement of highway.
Methods of Conducting Volume
Traffic volume counts are conducted using two basic
1. Manual
2. Automatic
 Manual Methods:
 One or more persons recording observed vehicles
using a counter.
 More than one counter can be used for turning
movements at intersections and types of vehicles.
One counter can be used for each type.
 Disadvantages:
Labor-intensive thus can be expensive.
Subject to the limitations of human factor.
Can not be used for long periods of counting.
Methods of Conducting Volume
 Automatic Method
some automatic counters use counting
methods that involve the laying on the
 surface detector (e.g. pneumatic road
tubes) (phoenix vehicle traffic classifier).
 Subsurface detectors (e.g. magnetic or
electric contact devices) (Hi-Star NC-90A)
 These detect the passing vehicles and
transmit the information to a recorder
placed at the side of the road
Types Volume Counts
Depending on the use of data, different types of
counts available:
1. Cordon:
 For information on vehicle accumulation within an
area such as CBD during a specific time
 Area of study is cordoned off by an imaginary closed
loop (cordon area)
 Intersection of each street crossing the cordon line is
taken as a count station.
 Volume counts of vehicle or persons entering and
leaving the cordon area are taken.
 Information collected is useful for planning parking
facilities, updating and evaluating traffic operational
techniques, and making long-range plans.
Types Volume Counts
2. Screen Line Counts:
 The study area is divided into large sections by
running imaginary lines (screen lines) across it.
 Screen lines Some times may be natural or man
made barriers (rivers, railway tracks).
 Traffic counts are then taken where roads cross
the screen line.
 Screen lines are chosen such that they are not
crossed more than once by the same street.
 It facilitate the detection of variations in the
traffic volume and traffic flow direction due to
changes in the land-use patterns of the area.
Types Volume Counts
3. Intersection Counts:
 Are taken to determine the vehicle classification of
through at turning movements at intersections.
 Data used to determine phase length and cycle times
for signalized intersections, and in design of
improvements to intersections.
4. Pedestrian Volume Counts:
 Are made at locations such as subway stations,
midblocks, and crosswalks.
 Used for evaluating of existing or proposed
pedestrian facilities (pedestrian overpasses or
 Can be conducted using manual counters.
 Also can be used for crash analysis, capacity
analysis, and in determining minimum timings at
signalized intersections.
Types Volume Counts
6. Periodic Volume Counts:
 AADT requires continuous volume counts…
expensive and not always feasible.
 To make reasonable estimate of traffic volume
characteristics on area wide bases…periodic
count are conducted (range from 15 minutes to
 Three types of periodic volume counts:
 Continuous Counts: uses permanent stations
and highways must be properly classified. Used
for AADT, daily, weekly, and seasonal variation
in traffic volumes
 Control Counts:
 Coverage Counts
Types Volume Counts
6. Periodic Volume Counts:
 Control Counts:
control stations are strategically located.
Used to count for seasonal and monthly variations of
traffic characteristics so that expansion factors can be
determined which can be used to determine year
round average values from short counts.
Major control counts: taken monthly, with 24 hr
directional counts taken on at least three days (Tue,
Wed, Thur), and also on (Sat & Sun) to oibtain
information on weekends on major roads. Data
collected give information regarding hourly, monthly,
and seasonal variations of traffic characteristics
Minor control counts: are five-day weekday counts
taken every other month on minor roads.
Coverage Counts
Types Volume Counts
6. Periodic Volume Counts:
 Coverage Counts:
 Are used to estimate ADT, using expansion
factors developed from control counts.
 Study area divided into zones with similar
traffic characteristics.
 A 24-hr non-directional weekday count is
taken at least once every four years at
each coverage station.
 Data indicate changes in area wide traffic
Traffic Volume Data Presentation
Data collected from traffic counts can be presented in
several ways depending on the type of counts
conducted and primary use of data.
Traffic flow maps:
 show volumes on individual routes.
 The volume of traffic on each route is presented by
the width of a band.
 See Figure 4.12
Intersection summary sheet: See Figure 4.13
Time-Based distribution chart: See Figure 4.14
Summary Tables: See Table 4.4
Example on Summary of Traffic
Volume Data for A Highway Section
Passenger Cars
Two-Axle Trucks
Three-Axle trucks
Other Trucks
Traffic Volume Characteristics
 Traffic volume varies from hour to hour, from day to
day, and from month to month.
 It was identified that this variation is repetitive and
 These characteristics can be used for planning and
projections for other time and place, and to estimate
the accuracy of traffic counts.
 See Figure 4.14
 Monthly variations (Fig. 4.14a):
 Low in Jan. & Feb. because of winter weather.
 Peak is during August due to vacation traffic.
 This indicate that traffic counts during these two
periods can not be representative of the average
annual traffic
Traffic Volume Characteristics
 Daily variations (Fig. 4.14b):
 Similar volumes on (Tue., Wed., and Thu.).
 Peak is observed on Friday.
 Indicating that when short counts are planned,
they should be held on (Tue., Wed., and Thu.).
 When planning for weekend counts use Fridays
and Saturdays.
 Hourly variations (Fig. 4.14c):
 Very low volumes between (1:00 and 5:00 am).
 Peak volumes between (8:00 and 9:00 am)
(12:00 -1:00 pm) and (4:00 – 6:00 pm).
 Work trips are the primarily responsible for the
Sample Size (Determination of
Number of Count Stations)
 The min. sample size depend on the precision level
desired, which is commonly used as 95%.
 When sample size is less than 30 and selection of
counting station is random…. Student (t) distribution
may be used to determine sample size of each class
of highway links.
 t-distribution is unbound, with a mean of zero, and
variance the depends on degrees of freedom (n).
 Degrees of freedom is a function of the sample size
[n = N-1].
 The variance of (t-dist.) is ( n/ n-2) which indicate
that as (n) approach infinity the variance approach 1.
 Confidence levels for t-dist. are given in Appendix A.
Sample Size (Determination of
Number of Count Stations) Cont.
 The min. sample number is given by:
N = [ (t2 a/2, N-1 (S2/d2)) /(1+(1/N) (t2 a/2, N-1)(S2/d2))]
n: min. number of count locations required.
t: value of the student t-dist. With (1-a/2) confidence
level and (N-1) degrees of freedom.
N: total number of links (population) from which a
sample is to be selected.
a: significance level.
S: estimate of the standard deviation of the link volumes.
d: allowable range of error.
 To use the above equation estimates of the mean and
std. dev. Of the link volumes are required. Which can
be found be taking volume counts at few links or by
using known value of other similar highways.
Sample Size (Determination of
Number of Count Stations) Cont.
 The FHWA suggested that the
Highway Performance Monitoring
System (HPMS) sample design is
more realistic to use.
 HPMS: is a stratified simple random
sample based on AADT.
 The data are stratified by:
 Type of area (rural, urban,…)
 Functional class.
Adjustment of periodic Counts
 Expansion factors are used to adjust periodic counts,
which can be determined either from continuous
count stations or from control count stations.
 Expansion factor from continuous count station:
1. Hourly Expansion factor (HEF)
2. Daily Expansion Factor (DEF)
3. Monthly Expansion Factor (MEF)
1. HEF = (total volume for 24-hrperiod / volume for
particular hour)
 Used to expand counts of durations shorter than 24hrs to 24 hrs volumes by multiplying the hourly
volume for each hour during the count period by the
HEF for that hour and finding the means of these
Adjustment of periodic Counts
2. DEF = (Average total volume for week / Average
volume for particular day)
 Used to determine weekly volumes from counts of 24hr duration by multiplying the 24-hr volume by the
3. MEF = (AADT / ADT for particular month)
 The AADT for a given year may be obtained from the
ADT for a given month by multiplying this volume bt
the MEF.
 See Table 4.5 to 4.7 for examples on expansion
 See Example 4.5