How many times have you traveled through some country or town roads to find these mysterious black tubes placed right on the tarmac? I’m sure quite a few. And a lot of us have wondered what these things are. Well, for starters, they’re called “pneumatic road tubes”. Unlike what some of you may have thought, they aren’t left their by mistake or in negligence. They’re put there deliberately, and serve a very important purpose.
Among other things, they help decide the budgets for city’s transportation network and road fixes.
Not always, but sometimes, these things do track the speed of vehicles. However, that’s not to dole out speeding tickets. It’s to understand the speed of traffic flowing through those roads. That’s how they’re used to determine if any changes need to be made in the speed limit for that part of the road. Or, if the enforcement of the speed limits needs to be stricter.
So, how do they exactly work?
The mechanism in these tubes let them keep a count of every vehicle that passes over them. That’s how they’re used to assess the traffic over the road, and various other parameters concerning it, like frequency during various times of the day or week, volume, etc. The U.S. Department of Transportation provides a more comprehensive and technical explanation:
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.
Why are there two of them placed closely sometimes, then?
Placing two tubes closely lets the system calculate the number of axles each vehicle passing over them have. This is important to understand the movement of large trailers over the road.
But why do they just disappear sometimes?
As we talked about earlier, these are primarily used to assess the infrastructure and budget needs for that road. They’re not a constant monitoring system, but a way to assess the current situation. That’s why, they’re mostly placed on a temporary basis. Most often, for just a day. When they’re placed temporarily, they’re generally pinched shut by a concrete nail on one end, and are fed into a counter device on the other end, that’s generally chained to a street post or sign. Sometimes though, these pneumatic road tubes may also be setup permanently.
Here’s a video that explains the whole thing in detail, if you care…