Have you ever wondered about those black hoses stretched out across the road? What do they do? How do they work? Where can I buy one? I can hardly go out in public anymore without someone asking me about those black “strips.”
Yep, some people refer to them as black strips. Also, black lines, hoses, tubes or very straight cracks. And, if you want to order some tubes for yourself, you will need to google: Ethylene propylene diene monomer road tube (but you did not hear that from me).
The rubber road tube, invented over 100 years ago, continues to be one of the most efficient and accurate methods for collecting traffic data. Although simple in operation, the data provided is quite impressive.
The road tube, connected to a device at the side of the road, records the precise time of each air pulse created by each passing vehicle. And because the tube is exactly perpendicular to the direction of traffic, each axle creates only a single pulse. So, for example, a passenger car creates a total of two pulses (one for each axle).
This means that for any given day (or hour or minute), we can know how many “axles” travelled on that segment of the road. And if the vehicles consist of 2-axle vehicles (i.e., cars), dividing the axle count by 2 yields the exact number of cars.
But what if there were TWO road tubes across the road? The available data gets impressive fast, hang on…
You will need to trust me on this, but when there are two road tubes, spaced at a specific distance (say 24”), we can determine much more. This arrangement allows us to determine, for each vehicle, it’s direction, speed, and type (car, truck, bus, semi, etc.). Determining direction is pretty straightforward — it’s the tube struck first. And because we know the spacing of the tubes and the time between pulses, we calculate speed. But how do we know if it’s a car, truck, bus, or semi?
Because we know the speed of each vehicle and the number of axles, we can determine the axle spacing. And the axle spacing (along with the number of axles) tells us if the vehicle was a motorcycle, car, truck, bus, semi, Amazon Prime, etc. — thirteen different categories to be exact.
Granted, there exist more “advanced” methods to collect traffic data such as radar, video, and cell phone MAC signatures (good topic for future column), but the amazing black rubber hose continues to be one of the best. The only drawback is they don’t stand up too well against snow plows…
Chet Skwarcan (traffic engineer, author, unique insights) with over 25 years of traffic engineering experience he continues to solve (or prevent) traffic problems. He can be reached at Chet@TrafficEngineering.com