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Director's Office
Municipal Plaza Building
114 W. Commerce
San Antonio, Texas 78205
Main Line: (210) 207-8022
Fax: (210) 207-4406

Traffic Studies

Traffic studies involve the collection and analysis of traffic data to aid engineers in their decision-making and planning processes. They are valuable for many reasons: they provide historical information about a location, they aid engineers when planning improvements, and they help identify problems and their level of severity. A traffic study typically involves collecting Traffic Counts, an Accident Investigation, and a Field Investigation.

Traffic Counts

Like San Antonio, most municipalities collect traffic counts using battery-powered traffic counters with road tube sensors, which are connected to a traffic counter. When a vehicle’s axle passes over the hoses, a pulse of air is sent to an air switch in the counter. The counter records the exact time of each axle hit. This is called collecting raw data, which can be analyzed to indicate the volume of traffic traveling the street, the vehicle speeds, type of vehicle and time gaps in traffic.

Road Tube Counts

Turning movement counts are traffic counts taken at intersections which provide the volume of traffic by vehicle movement. This helps identify the predominant and minor traffic patterns at intersections and helps determine the proper timing for the traffic signal under existing traffic conditions. Since traffic signals are timed according to the volume of traffic performing each movement, these counts provide the engineer with the information needed to optimize the timing of the traffic signal.

Accident Investigation

The objectives in performing accident analyses are to:

  • help identify high accident locations,
  • evaluate alternative corrective treatments,
  • determine the effectiveness of corrective treatments in order to compare before and after accident occurrences, and
  • aid in the planning process for the development of future projects.

A concern for traffic safety and/or a complaint concerning reoccurring accidents at a particular facility may be cited as reasons to make changes. In this case, a review of accident records for the location in question will occur. Accident problems will likely be revealed by abnormal trends in the accident history. It is these trends that investigators search for and use to evaluate and recommend corrective treatments.

Traffic related improvements can be accomplished by reviewing accident frequency and the accident rate. Although some use these terms interchangeably, there is a distinct difference. Accident frequency is simply the number of accidents occurring at a given location over a time period. Although this can be useful information especially when determining if traffic controls are warranted, this method fails to consider other factors such as the volume of traffic using a particular facility, the facility type, and geometrics. An accident rate, however, takes these items into consideration. For example, an intersection which experiences three accidents per year may be considered hazardous if only 100 vehicles per day travel through the intersection. Conversely, the same accident frequency at an intersection with 15,000 entering vehicles per day would be considered safe. Accident rates at intersections are reported in terms of a number of accidents per million entering vehicles. For roadway sections, accident rates are indicated by a number of accidents per million vehicle-miles of travel. Accident rates are especially useful when the rate at a particular facility is compared to the average rate for similar facilities throughout a geographical area.

Field Investigation

A field investigation is completed on nearly every request received for new traffic control devices. The only time a field investigation may not be necessary is for maintenance items such as traffic sign replacements. When a complaint is received, a field investigation is necessary to verify problems and help the investigator search for treatments. Many times, more than one field investigation may be necessary depending on the nature of the request.

Merely observing traffic and street geometry can be insightful. In fact, a field investigation is often the most informative aspect of the traffic study and can help identify changes which have occurred over time. Field investigations may also provide additional information that is not readily accessible within the records on file for the location being studied.

The following are a few examples of requests requiring a field investigation:
  • Complaints about inadequate sight distance
  • Complaints about curves which are not safe at the posted speed limit
  • Requests for stop signs, yield signs, speed limit signs, and other sign installations
  • Requests for the installation of guardrails
  • Requests for speed humps or other traffic calming features
  • Requests for traffic signals
  • Requests for parking restrictions
  • Requests for removal of any existing traffic control device