Traffic Studies - Precision Traffic and Safety Systems

Traffic Studies

A traffic study consists of a comprehensive investigation of existing physical and operating conditions. Analysis of the study data provides insight into possible remedial measures, if any. Remedial measures may include various traffic control measures, such as speed zoning, channelization, signing, traffic signals, safety lighting, or a combination of these.

A complete traffic study for a proposed traffic signal or flashing beacon installation requires the collection of sufficient data on the physical, traffic, and operational characteristics of the intersection. Some of the data can be difficult and time consuming to collect. Examples of the types of data typically collected are:

  • Condition diagram – A study should be made of existing conditions at the study location and along each approach to it. Where the location is open to traffic, the study should include existing traffic control devices (signs, signals, markings, speed zones, etc.).
  • Location map– A city or area location map should be prepared to show the relationship of the proposed installation to other traffic signals, highways, business areas, and traffic generators.
  • Photographs– Photographs should be made of all approaches to an intersection, ramp, or any other location being studied for a traffic signal. Generally, only one photograph is necessary for each approach if the photograph is taken from a position approximating that of a passenger car driver approaching the intersection. This would normally be from 46 to 91 m (150 to 300 feet) from the study location. More than one photograph may be required for a particular approach to fully illustrate problems at the location. Peripheral features may also be helpful in defining any operational problems. The location and approach view direction of each photograph should be indicated on the photograph itself.
  • Information– Accident (crash) information for traffic studies is obtained primarily through mainframe computer reports.
  • Vehicular and pedestrian traffic counts– Up-to-date traffic and pedestrian volume counts reflect the characteristics of traffic. These volume counts, when compared to the established warrants, help determine the appropriate type of traffic control device, if any.
Types of Counts
A volume count analysis may use the following types of counts:
  • Vehicle counts at existing locations estimated counts at locations under construction and projected counts for future locations
  • Pedestrian counts
  • School pedestrian counts.
Vehicle Counts at Existing Locations
Vehicle count at an existing location should include the number of vehicles entering the location on each approach. Counts are recorded as vehicles cross the stop bar and enter the intersection. Tallies should be recorded for each quarter hour for the duration of the count. Ideally, counts are conducted on a “representative day”. The duration of the count should be 16 consecutive hours. This time span should contain the greatest percentage of traffic during the 24-hour time period. Traffic patterns, such as when the highest vehicle and pedestrian volumes occur, should help determine the beginning and ending times for the count. These patterns may vary from one location to the next.
Counting Techniques
With passenger or commercial vehicles, manual turning movement counts are always preferable, as they provide both the basic data for justification as well as detailed guidance for design. When 16-hour, machine-recorded traffic counts are used for traffic signal studies, they should be supplemented with manual counts for two hours of the morning peak and two hours of the afternoon peak periods.
Recording Manual Counts
Manual traffic counts may be recorded on either a volume summary sheet or the vehicle volume field sheet.
Estimated and Projected Counts
Using the projected anticipated average daily traffic ADT volumes, the following general guidelines can be used to obtain an estimate of vehicle count data:
  • The maximum 8-hour volume is generally between 50 percent and 60 percent of the ADT, with the average being approximately 52 percent. In an urban area with a high ADT, the percentage is generally between 55 and 60 percent.
  • The average hourly volume of the maximum 8 hour volume is generally between 6 percent and 8 percent of the ADT maximum 8 hour volume divided by eight.
  • The lowest hourly volume (eighth highest hour) of the maximum 8 hour volume is generally between 5 percent and 6 percent of the ADT with an average value of approximately 5.5 percent. It is also approximately 80 percent of the average hourly volume or 10 percent of the maximum 8 hour volume. This value is the basis for comparing the anticipated volumes with the volume warrants for signalization.
  • The lowest hourly volume (eighth highest hour) for a “grid system” of existing signals within a city is assumed to be 5.0 percent of the ADT.
  • The peak hour volume (highest hour) is generally between 6 percent and 10 percent of the ADT. The lower values are generally found on roadways with low volumes. The average value is approximately 8.4 percent of the ADT.
Pedestrian Counts
  • Pedestrian volume counts for each cross walk should be made during the same period as the vehicle volume count. Tallies should be recorded for each quarter hour for the duration of the count.
  • Pedestrian counts are not required in sparsely settled rural areas or at other locations where it is apparent that pedestrian movement is negligible. The signal installation must comply with the latest version of the Americans with Disabilities Act.
School Pedestrian Counts
School pedestrian counts should be made on a normal school day during the hours of greatest crossing activity. Obtaining the necessary count information for a school crossing study involves:
  • Counting the number of vehicles
  • Determining the length and spacing of gaps in the traffic stream
  • Measuring the width of the street
  • Counting the number of pedestrians crossing the street during each gap in the traffic stream.
  • Approach speeds – The 85th percentile speed on each of the approaches to the location should be determined. Where approaches are controlled by a STOP sign or traffic signal at the time of the study, the speed check must be conducted far enough away from the location to obtain a speed not affected by the sign or signal.
  • Traffic Survey Count Analysis – Vehicle and pedestrian count data must be tabulated and checked against established traffic signal warrants. The vehicle data are recorded on the vehicle volume summary sheets or the vehicle volume field sheets. The pedestrian data are recorded on the same summary sheet.
  • Intersection delay study – Typically, an intersection delay study is conducted at intersections or major driveways where congestion problems exist. This study is considered as a detailed investigation of the stopped-time delay conditions at an intersection being evaluated for signalization.

When the traffic signal study is complete, the information is tabulated and checked against the traffic signal warrants set forth in the local city or municipality.