You’re driving around on a relatively well-paved road. The road has a double-yellow line, suggesting that it’s not a road in which kids should be playing or where there’s a high likelihood of a pedestrian suddenly jutting out into the street. You can drive on the faster side without too much additional risk.
But then, as you speed on at 35 miles per hour, you hear the little “ba-dump ba-dump” as you glide over those two little black bumps in the road.
Another thing, the black tubes, are actually called “pneumatic road tubes”. Most people think they’re used for counting cars, which they are, but they have many more functions than that.
What are those odd black tubes in the road used for? Fortunately, I can help you with this one. And no, it’s not a goofy elaborate watering system.
Seemingly low-tech, these tubes collect vehicle data over time (we usually conduct 7-day counts to ensure we understand the difference in travel behavior during peak weekdays, usually Tuesday through Thursday, while also including the weekend; typically, there is a steep drop-off in vehicle volumes between weekday and the weekend).
Here is how speed studies are generally conducted:
- A street is selected for the speed/volume study by city staff or a resident. The street is analyzed for the best place to place the tubes; generally, mid-block locations away from intersections or other obvious impediments that aren’t representative of the streetscape are preferred.
- The tubes are nailed into the roadway by a trained technician. Precise, parallel placement of the tubes is critical to ensure accuracy.
- Vehicles travel over the tubes for the duration of the study. As vehicles travel over the tubes, blasts of air are sent to the receiver box and the data is stored in the box until the technician picks up the equipment. With a known distance between the parallel tubes, speeds are calculated by measuring the time difference between the front axle hitting the first and second tubes.
- After the field component is completed, the technician retrieves the equipment and compiles the data to send to staff.
- Staff receive the data, interpret the results, and communicate a summary to residents (often in the form of a Traffic Action Plan).
According to the U.S. Department of Transportation, pneumatic road tube sensors send a burst of air pressure along a rubber tube when a vehicle’s tires pass over the tube. The pressure pulse closes an air switch, producing an electrical signal that is transmitted to a counter or analysis software. The pneumatic road tube sensor is portable, using lead-acid, gel, or other rechargeable batteries as a power source.
It’s a pretty cool system if you think about it. It can be set up permanently or temporarily, so very flexible.
Now you know what those crazy tubes are for. No more wondering.
Watch the video below: