traffic volume study methodology_slides

8/10/2019 Traffic Volume Study Methodology_Slides

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Traffic Volume Studies

Engineers often use counts of number of vehicles orpedestrians passing a point, entering an intersection, orusing a particular facility such as travel lane, crosswalkor sidewalk.

Counts are usually samples of actual volumes, althoughcontinuous counting is also sometimes performed.

Sampling periods may range from a few minutes to a

month or more.

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The two basic methods of counting traffic are:

Manual observation

Automatic recording


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Manual Observation

Many types of counts require classification thatare obtained more easily and accurately withtrained observers.

Examples include vehicle occupancy,pedestrians, turning movements, and vehicleclassifications.

Other reasons for conducting manual countsare time and resources.

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Traffic Volume StudiesManual Observation

Typical equipment needed for manual countsare:

Tally Sheets, or

Mechanical Count Boards, or

Electronic Count Boards


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Manual Observation

Mechanical Count Board

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Traffic Volume StudiesManual Observation

Electronic Count Board


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Manual Observation

Manual traffic counting requires trained observers.

They must be relieved periodically to avoid fatigue anddegraded performance.

Breaks of 10 to 15 minutes should be scheduled at leastevery 2 hours.

If data collection period is more than 8 hours, breaks of30 to 45 minutes should be allowed every 4 hours.

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Traffic Volume StudiesAutomatic Recording

The automatic counting methodinvolves

the laying of surface detectors (such as

pneumatic road tubes) or subsurface

detectors (such as magnetic or electronic

contact devices) on the road. These detect

the passing vehicle and transmit the

information to a recorder, which is connectedto the detector at the side of the road.


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Automatic Recording

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Traffic Volume Study

Daily Volumes and Their Use

A common time interval for volumes is aday.

Daily volumes are frequently used as thebasis for highway planning and generalobservations of trends.

Traffic volume projections are oftenbased on measured daily volumes.


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Traffic Volume Study Daily Volumes and Their Use (Contd..)

There are four commonly used daily volume parameters:

1. Average Annual Daily Traffic (AADT): is the average 24-hrtraffic volume at a given location over a full 365-day year.

2. Average Annual Weekday Traffic (AAWT): is the average 24-hr traffic volume occurring on weekdays over a full 365-dayyear.

3. Average Daily Traffic (ADT): is an average 24-hr volume ata given location for some period of time less than a year,but more than one day.

4. Average Weekday Traffic (AWT): is an average 24-hr traffic

volume occurring on weekdays for some period less thanone year.

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Traffic Stream Parameters


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Traffic Stream Parameters Hourly Volumes and Their Use

While daily volumes are useful in highway planning,they cannot be used alone for design or operationalanalysis purposes.

Traffic volume varies considerably during the courseof a 24-hr day, usually with periods of maximumvolume occurring during the morning and eveningcommuter rush hours.

The single hour of the day that has the highesthourly volume is referred to as the peak hour.

Traffic volume within this hour is of greatest interestto traffic engineers in design or operational analysis.

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Hourly Volumes and Their Use (contd)

The peak hour volume is generally a directional volume andmay be viewed as a directional design hour volume (DDHV).

Directional Design Hour Volume may estimated using thefollowing relationship

DDHV = AADT x K x D

K = proportion of daily traffic occurring during the peakhour, expressed as a decimal, and

D = proportion of peak hour traffic traveling in thepeak direction, expressed as a decimal.

K often represents the proportion of AADT occurringduring the thirtieth highest peak hour (30 HV) of theyear.


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Traffic Stream Parameters Hourly Volumes and Their Use (contd)

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Hourly Volumes and Their Use (contd)


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Types of Counts

Different types of traffic counts arecarried out, depending on theanticipated use of the data to becollected. They are: Cordon Counts

Screen Line Counts

Intersection Counts

Pedestrian Volume Counts, and

Periodic Volume Counts (Continuous,Control, and Coverage Counts)

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Traffic Volume StudiesTypes of Counts

1.Cordon Counts

When information is required on vehicle accumulationwithin an area, a cordon count is undertaken.

The area for which the data are required is cordonedoff by an imaginary closed loop.

The intersection of each street crossing the cordonline is taken as a count station where volume countsof vehicles and /or persons entering and leaving thecordon area are undertaken.


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Types of Counts

Information obtainedare useful forplanning parkingfacilities, updatingand evaluating trafficoperationaltechniques, andmaking long rangeplans for freeway

and arterial streetsystems.

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2. Screen Line Counts The study area is divided into

large sections by runningimaginary lines across it.

Traffic counts are taken at eachpoint where a road crosses thescreen line.

Data collected facilitates thedetection of variations in the

traffic volume and traffic flowdirection due to the changes inthe land-use pattern of thearea.


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Types of Counts

3. Intersection Counts

Intersection counts are taken to determine

vehicle classification, through movements and

turning movements at intersections.

Data collected are mainly used in determining

phase lengths and cycle times for signalized

intersections and also for channelization at the

intersections.

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4. Pedestrian Volume Counts

Volume counts of pedestrians are made

at locations such as mid-blocks,

crosswalks, and transit stations.

Counts are usually made to evaluate theexisting or proposed pedestrian

facilities.


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Types of Counts

5. Periodic Volume Counts

In order to obtain certain traffic volumedata, such as AADT, it is necessary toobtain data continuously.

However, it is not feasible to collectcontinuous data on all roads because of

the cost involved.

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To make reasonable estimates of annualtraffic volume characteristics on anarea-wide basis, different types ofperiodic counts with count durationsranging from 15 minutes to continuous,are conducted.

Periodic volume counts are used tocalculate expansion factors needed toestimate the annual traffic volume.


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Traffic Volume Studies Expansion Factors from Continuous

Count Stations

Hourly Expansion Factor (HEF)

hourparticularforvolume

periodhrforvolumetotalHEF

=

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Expansion Factors from ContinuousCount Stations


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Count Stations

Daily Expansion Factor (DEF)

dayparticularforvolumeaverage

weekaforvolumetotalaverageDEF =

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Count Stations

Monthly Expansion Factor (MEF)

monthparticularforADT

AADTMEF =

30




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Traffic Volume Studies Calculating AADT using expansion factors

A traffic engineer urgently needs to determine theAADT on a rural primary road that has the volumedistribution characteristics shown in previous Tables4.6, 4.7, and 4.8. He collected the data shown belowon a Tuesday during the month of May. Determinethe AADT of the road.

Time Period # Vehicles7:00 8:00 AM 4008:00 9:00 AM 5359:00 10:00 AM 65010:00 11:00 AM 71011:00 12:00 Noon 650

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Calculating AADT using expansion factors


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Types of Counts

Short Counts

In taking intersection counts with people in the field,even when automated recording equipment is used, itis common for an observer to take short breaks duringthe counts and/or alternate attention betweenapproaches, movements, or lanes.

This is done for practical reasons, including the costassociated with crew size and equipment.

The need to do short counts and/or alteration presentsan interesting estimation problem.

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Expanding Short Counts


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Presentation

The data collected from traffic volumecounts may be presented in severalways depending on the type of countconducted and the primary use of thedata. They are:

Traffic Flow Maps

Intersection Summary Sheets

Time-Based Distribution Charts Summary Tables

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Traffic Volume StudiesPresentation

Traffic Flow Maps


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Presentation

IntersectionSummary Sheets

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Traffic Volume StudiesPresentation

Time-BasedDistributionCharts


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Presentation

Summary Tables

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Volume/Demand/Capacity

Volume on a road is the number of vehiclespassing the measurement point during a

specified time interval.

Demand is a measure of the number ofvehicles (or passengers, or persons) waiting

for service in the given time period.

Capacityis the maximum number that canreasonable be expected to be served in the

given time period.

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