10543, 10544, 10545...

Throughout the course of my education and experience in the traffic world, I've run across some things which I think are interesting tidbits. I'll try to share them with you from time to time.

Here's one. Ever seen those black tubes that are stretched across a road? Even if you don't see them, you might hear the bump as you drive over them. You may have assumed, correctly, that your vehicle was just counted.

Before I knew how they worked, I always thought that compressing the tube somehow made two wires touch and that's how the cars were counted. What actually happens is that the tube is just a hollow rubber tube that is tied in a knot at one end and the other end is connected to a counter, shown in the picture here with Mr. Traffic Counter Guy. When your tire rolls on the tube, it compresses the air inside the tube and causes a "puff" at the counter. The counter has a sensor for this puff and uses it to count the cars. Of course, you have to divide the total by two, to account for the front and back tires.

Here in NC, there is a specific unit in the Department of Transportation that regularly counts the traffic on most of the roads throughout the state. They provide these counts on big maps in the form of Annual Average Daily Traffic (AADT). This information is used in many ways, such as deciding which roads get repaved and which intersections get upgrades like traffic lights. Another use is here in the traffic safety research world, where we take the total number of crashes on a particular road and divide it by the AADT to look at the crash rate per vehicle.

So, anything on the roads you've been wondering about?