What are Those Two Black Cables on Washington Roads?

Have you seen two black cables lying across a road in Washington? I’ve recently seen this in a couple of locations around my hometown. In both cases, they are placed with a small spacing between the two black wires, or cables.

What are they? 

What is it measuring? 

Are they trying to catch you speeding?

Yes and no on the speeding part.

They're called "pneumatic road tubes"

The two black cables are pneumatic road tubes that gather information for the city or county. According to Mental Floss, a pair of tubes can gather data on the direction, speed, and class of vehicles passing over the area in a given time.

How do they work?

The United States Department of Transportation explains that:

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.

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Small lego cars with speed information
Nick Moore (via YouTube)

The information from the pneumatic tubes tells city or county planners: 

  • The number of vehicles that pass over them
  • The size of the vehicle
  • The length (is it a passenger vehicle or an 18-wheeler semi?)
  • What’s the weight?
  • What speed are the vehicles traveling on that stretch of road?

Sometimes the cables are installed for a short time (such as 24 hours), while others are left there on the road for days or even weeks.

Information that is collected can help state or city governments better improve roadways.


Here is a video explaining how road planners use the info:


Now that you know, where will you drive next? Check out these amazing back road trips in Washington.

8 Really Fun Day Road Trips To Take in Washington State

Ready for a day of fun and adventure? Here's a list of eight amazing road trips you can take in Washington state no matter the time or budget. 

Gallery Credit: Rik Mikals

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