Traffic Volume Study

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Traffic Volume Studies

Traffic Volume Studies

Engineers often use counts of number of vehicles or pedestrians passing a point, entering an intersection, or using a particular facility such as travel lane, crosswalk or sidewalk. Counts are usually samples of actual volumes, although continuous counting is also sometimes performed.

Manual Observation

Typical equipment needed for manual counts are: Tally Sheets, or



Mechanical Count Boards, or

Sampling periods may range from a few minutes to a month or more.

Electronic Count Boards
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Traffic Volume Studies

Traffic Volume Studies
The two basic methods of counting traffic are: Manual observation Automatic recording

Manual Observation

Mechanical Count Board
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Traffic Volume Studies
Manual Observation

Traffic Volume Studies
Manual Observation

Many types of counts require classification that are obtained more easily and accurately with trained observers. Examples include vehicle occupancy, pedestrians, turning movements, and vehicle classifications. Other reasons for conducting manual counts are time and resources. 3

Electronic Count Board
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Traffic Volume Studies
Manual Observation

Traffic Volume Study
Daily Volumes and Their Use

Manual traffic counting requires trained observers. They must be relieved periodically to avoid fatigue and degraded performance. Breaks of 10 to 15 minutes should be scheduled at least every 2 hours. If data collection period is more than 8 hours, breaks of 30 to 45 minutes should be allowed every 4 hours.

A common time interval for volumes is a day. Daily volumes are frequently used as the basis for highway planning and general observations of trends. Traffic volume projections are often based on measured daily volumes.

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Traffic Volume Studies
Automatic Recording

Traffic Volume Study
Daily Volumes and Their Use (Contd..)
There are four commonly used daily volume parameters:

The automatic counting method involves 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 connected to the detector at the side of the road.

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Average Annual Daily Traffic (AADT): is the average 24-hr traffic volume at a given location over a full 365-day year. Average Annual Weekday Traffic (AAWT): is the average 24hr traffic volume occurring on weekdays over a full 365-day year. Average Daily Traffic (ADT): is an average 24-hr volume at a given location for some period of time less than a year, but more than one day. Average Weekday Traffic (AWT): is an average 24-hr traffic volume occurring on weekdays for some period less than one year.

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Traffic Volume Studies
Automatic Recording

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 operational analysis purposes. Traffic volume varies considerably during the course of a 24-hr day, usually with periods of maximum volume occurring during the morning and evening commuter “rush” hours. The single hour of the day that has the highest hourly volume is referred to as the “peak hour”. Traffic volume within this hour is of greatest interest to traffic engineers in design or operational analysis. 13

Traffic Stream Parameters
Hourly Volumes and Their Use (contd…)

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Traffic Volume Studies

Traffic Stream Parameters
Hourly Volumes and Their Use (contd…)
The peak hour volume is generally a directional volume and may be viewed as a directional design hour volume (DDHV). Directional Design Hour Volume may estimated using the following relationship DDHV = AADT x K x D K = proportion of daily traffic occurring during the peak hour, expressed as a decimal, and D = proportion of...
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