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Traffic Monitoring: A Guidebook

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2. Traffic Monitoring Basics

Who Needs Traffic Monitoring?

Traffic monitoring is used by various federal, state, and local governmental agencies, as well as private businesses which require information pertaining to travel patterns in their respective regions. Federal agencies, such as the Federal Highway Administration (FHWA) for example, require states to collect data used to estimate total vehicle miles of travel (VMT) in order to determine the apportionment of Federal-Aid funds. States and local government agencies will typically collect additional data that is used to determine pavement design, planning of future roadways, air quality, and maintenance needs.

What Is Traffic Monitoring?

Traffic monitoring is the collection of information to describe the use and performance of the roadway system based on the following:

  • Traffic volumes - number of vehicles;
  • Vehicle classification (VC) - type of vehicle (e.g., car, motorcycle, truck, etc.);
  • Vehicle occupancy - number of riders in a vehicle;
  • Truck weight volumes - number of heavy vehicles by weight classification;
  • Pedestrian volumes - number of people walking; and
  • Bicycle volumes - number of bicycle riders.

Traffic Monitoring carried out to collect data on the number of vehicles and/or pedestrians that pass a point on a roadway facility during a specified time period. This same collected data may also be put into categories, for example:

  • Vehicle occupancy rates (number of passengers divided by number of vehicles);
  • Directional movement of vehicles (into or out of park facility); and
  • Pedestrian age.

Where Is Traffic Monitoring Needed?

Traffic monitoring is needed in various settings and locations depending on the use for the information that is collected. For example, traffic monitoring can be performed on the following:

Highways/Freeways
  • To determine the planning of highway activities;
  • To measure the current traffic demand; and
  • To evaluate existing traffic flow.
Arterial/Local Roads
  • To determine intersection improvements;
  • To change a speed limit on a particular road; and
  • To develop parking regulations.
Parking Areas
  • To determine the need for additional access locations;
  • To determine the location of pedestrian crosswalks, Parks and Recreational Areas;
  • To determine the variability in usage of park roads;
  • To assist in developing schedules due to change in facility usage; and
  • To optimize traffic routing.

When Is Traffic Monitoring Performed?

Traffic monitoring is measured at various increments of time depending on the intended use of the collected information. The choice of time periods can vary from as little as 5 minutes to as much as a year and are derived from the following:

Time-of-Day
  • Minute intervals - 5, 15, and 30 minute intervals are common;
  • Peak hour volume (PHV) - highest traffic volume in a consecutive 60 minutes;
  • Peak period - highest traffic volume in a consecutive 180 minutes;
  • Average daily traffic (ADT) - average of 24 hour counts collected over a number of days greater than one but less than a year;
  • Average annual daily traffic (AADT) - average of 24 hour counts collected every day in the year; and
  • Off-peak hours - traffic volumes not within the peak period hours of a day.
Day-of-week
  • Weekday (Monday through Friday); and Weekend (Saturday and Sunday).
Seasonal
  • Winter, spring, summer, fall.
  • Holidays
  • July 4, Labor Day, Memorial Day, etc. Spring break, Thanksgiving break, etc.

Note: This information may be collected over the entire period of time or a "sample" that represents the specific time period. "Sampling" requires specific guidelines that should be followed to ensure accurate collection of traffic information.

Why Is Traffic Monitoring Performed?

Traffic monitoring is performed in order to provide the factual information needed to make proper assessments and appropriate decisions related to travel patterns. Traffic monitoring assists in the following types of decisions and examples of applications:

Planning Decisions
  • To determine if a traffic signal is needed at an intersection;
  • To determine traffic control devices needed, such as speed limit signs; and
  • To determine the functional classification of a roadway to reflect principal use.
Design Decisions
  • To determine the number of lanes needed on a roadway;
  • To determine the load weight of a bridge to be built; and
  • To determine where improvements are needed.
Construction Decisions
  • To determine lane closures to minimize travel impacts; and
  • To determine routing alternatives around closed roadways.
Operational Decisions
  • To determine the signal phasing at an intersection;
  • To determine maintenance schedules for pavement patching and repair; and
  • To evaluate air quality conditions.

How Is Traffic Monitoring Performed?

Traffic monitoring is performed using one of the following available techniques based upon the length of the sampling period. The available techniques include manual counting and mechanical or automated counting which utilizes specialized equipment to collect the traffic information.

Manual Counting
  • Trained observer at each location
  • Short sampling period (less than 10 hours of data at any location)
  • Lower cost to perform
  • Less effort to implement
Equipment Needs
  • Tally sheets
  • Mechanical or electronic count boards
  • Watch or stopwatch
Automated Counting
  • Automatic data collection equipment at each location
  • Longer sampling period (24 hours a day)
  • Higher cost to perform
  • More effort to implement due to equipment setup and retrieval
Equipment Needs
  • Portable counters (24 hours a day, up to several weeks)
  • Sensor detectors include pneumatic tubes, piezoelectric strips, tape switches, and temporary induction loop detectors.
  • Permanent counters (24 hours a day, 365 days a year)
  • Sensor detectors include induction loop detectors, bending plates, radar, microwave, and video imaging.

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