Did You Know: Counting Cars is Fundamental to Traffic Engineering

Posted by on Oct 20, 2015

As civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.

People frequently ask our traffic engineering staff, “Do you really sit on the side of the road and count cars?”  The answer to that is yes, but we also have many other ways to measure and record traffic on roadways and at intersections.

Sitting on the side of the road and counting cars is actually one of the initial tasks we use to train new employees on our traffic engineering team. It is a valuable exercise which helps them become familiar with conducting and interpreting traffic counts and understanding how they are used in a study or a design.

If you ever see one or more white vans with state license plates on them at an intersection, it is more than likely the Alabama Department of Transportation (ALDOT) counting traffic. Their counting procedures require them to count every single vehicle which passes through the intersection between 6:00am and 10:00pm.

Vehicles can also be counted using machines with road tube at specific locations. ALDOT has hundreds of permanent traffic count “stations” across Alabama on state, federal and major local roads that are constantly collecting vehicle traffic count data. At permanent traffic count stations, data can be collected through the use of in-pavement vehicle detectors or non-intrusive devices which detect vehicles through the use of radar technology.

The black tubes that go across the road are the first element of a system which is designed to collect traffic counts. Road tubes can also record individual vehicle travel speeds and vehicle type. The black tubes are stretched across the road perpendicular to the travel lanes and nailed in place with special asphalt or concrete nails. The end of the tube that goes on the road is capped. The open end of the tube is connected to a battery operated device placed away from the side of the road or in the median (usually chain-locked to a pole or sign post) which records the tube being crossed by a vehicle. When a vehicle crosses the tube, it forces air to travel through the tube and register a “hit” at the recording device. The recording devices record and timestamp every hit. When the count is complete, the device is taken to an office and connected to a computer where the recorded data can be processed and summarized.

Video recording is one of the increasingly common methods to count vehicles. The count location is recorded with a video camera, and the video is played back and manually counted at an office. Video recording is most helpful on projects that have many count locations involved. Video cameras can be used to capture data at more sites over a shorter period of time with less manpower than traditional manual human count methods.

Radar technology is also being used to collect traffic count data at permanent count stations. Unmanned aerial vehicles (a.k.a. drones) have been used in some areas for extremely large scale traffic counts, but at the present time they are more effective for general monitoring purposes associated with unique traffic conditions such as work zones, construction sites and special events.

So why is it important to count all of this traffic? The counts provide a baseline of traffic volume that we use to perform analysis on a roadway or intersection. Traffic counts are used to determine the number of lanes needed on a roadway. At an intersection, traffic counts are used to determine the number and length of turn lanes needed, and the appropriate traffic control measure (stop sign, traffic signal, roundabout, etc.).  If there is a traffic signal involved, traffic counts are critical to developing appropriate signal timing.  In addition to vehicles, pedestrian and bicycle count data is collected and used in order to properly accommodate pedestrians and cyclists into the analysis and design of a project.

There is a lot of information to record, process and analyze, and it’s important to know when a more traditional method or a newer method is best. With this combination of methods, we are able to get the most accurate picture of traffic so we can provide successful traffic engineering.

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