Why perform 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 the facility, they aid engineers in the decision-making process when planning improvements for a particular facility, and they help identify problems and their level of severity. Traffic studies are performed to provide information about a particular facility which aids in the decision-making process when changes in traffic control are being contemplated.

Elements of a Traffic Study:
A traffic study typically involves one or more of the following:

1. Collecting traffic counts (volume, speed, vehicle classifications, turning movement counts, etc.),
2. Reviewing accident data,
3. Field investigation.

Collecting Traffic Counts
Like San Antonio, most municipalities collect traffic counts using battery-powered traffic counters with road tube sensors. To collect counts, one or two rubber hoses (depending on the information needed) are stretched across the road and affixed. The hoses are then 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. The data are recorded in the counter’s memory and then downloaded to a computer in the office. Special computer software is used to analyze the data and display it in a useful format.

Depending on the layout of the hoses on the street, the data can be analyzed to indicate the volume of traffic traveling the street, the vehicle speeds, vehicle classifications (passenger car, long wheel based truck, buses, etc.), and gaps in traffic (the number of seconds between vehicles).

Road Tube Counts 

Turning Movement Counts (TMC)
Turning movement counts are traffic counts taken at intersections which provide the volume of traffic by vehicle movement. TMC’s help engineers identify the predominant and minor traffic patterns at intersections which helps in the planning process. For example, when a traffic signal is installed at an intersection, the engineer must determine the proper timing for the traffic signal under the existing traffic conditions. Since traffic signals are timed according to the volume of traffic performing each movement, TMC’s provide the engineer with the information needed to optimize the timing of the traffic signal.

Accident Investigation
An accident occurs following the failure of one or more of the driving components, resulting in death, bodily injury, and/or property damage. Although driver error is the primary cause of most accidents, accident analysis may be helpful in making improvements that reduce accident potential.

The objectives in performing accident analyses are as follows:

• to help identify high accident locations
• to evaluate alternative corrective treatments,
• to determine the effectiveness of corrective treatments subsequent to their implementation by comparison of before versus after accident occurrence,
• to aid in the planning process for the development of future projects.

A concern for traffic safety and/or a complaint about an accident problem 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 is a logical starting place. 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.

The identification of potentially hazardous locations can be accomplished by reviewing accident frequency and 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 some 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 very 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 some geographical area.

Field Investigation
The Traffic Engineering Division of Public Works performs a field investigation 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 very insightful. In fact, a field investigation is often the most informative aspect of the traffic study. Even if drawings are existing in the location files, field investigations help identify changes which have occurred over time or field changes which occurred at the time of construction that were not annotated on the plans. Field investigations provide much useful information.

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.

If you have a specific concern about traffic conditions on a city street, then you may communicate your concern to the City of San Antonio by calling 311.

Frequently Asked Questions (FAQs)
Each year, the city receives many inquiries from citizens regarding traffic safety and/or congestion. The following information may be helpful in understanding the abilities and limitations of certain traffic control devices. (Add links to all appropriate pages for topics below)

1. Does lowering the speed limit solve a speeding problem?



2. Should stop signs be used to control speeding?



3. How does an All Way Stop get installed?



4. What constitutes a visual obstruction?



5. Where can I find out whether a particular school zone is eligible for a school flasher or not?



6. Do marked crosswalks make it safer for pedestrians to cross the road?



7. Where can I find more information to request a speed hump for my street?



8. Do 'Slow - Children at Play' signs provide protection for children?



9.  What are facts and what is fiction about traffic signals?



10. Who do I call to report a malfunctioning traffic signal, downed traffic signs or another traffic operation problem:
    
    Please call the Traffic Operations Center at (210) 207-7765 OR 311.



11. Where can I report graffiti on traffic signs, a missing street name sign or report other problems?



12. What information is needed for requesting a temporary street closures or a temporary parking permit?

For customer service, please call 311.