# What are those cables you see in the road? **URL:** \ **Category:** Factual Questions\ **Created:** [August 2, 2003, 4:44am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502 "2003-08-02T04:44:07Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Ashkicker](https://avatars.discourse-cdn.com/v4/letter/a/4bbf92/32.png) [@Ashkicker](https://boards.straightdope.com/u/Ashkicker)\ **Post date:** [August 2, 2003, 4:44am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/1 "2003-08-02T04:44:07Z")
Does anyone know what those black cables you see in the road are? I figure they are either measuring the speed of the cars or the number of cars that pass over it. Just curious. ---
**Author:** ![Berkut](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@Berkut](https://boards.straightdope.com/u/Berkut)\ **Post date:** [August 2, 2003, 4:49am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/2 "2003-08-02T04:49:55Z")
They are used to measure the volume or speed of traffic. They are rubber hoses connected to a box with air switches. The vehicle passes over them, compressing the air and actuating the switch. There are some variables, such as the number of axles and such that they have to take into account. It’s a similiar setup to a service station bell hose. ---
**Author:** ![5-HT](https://avatars.discourse-cdn.com/v4/letter/5/f19dbf/32.png) [@5-HT](https://boards.straightdope.com/u/5-HT)\ **Post date:** [August 2, 2003, 5:16am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/3 "2003-08-02T05:16:14Z")
> [@](#): > > \*Originally posted by Ashkicker \* > \*\*Does anyone know what those black cables you see in the road are? I figure they are either measuring the speed of the cars or the number of cars that pass over it. Just curious. \*\* I’ve wondered the same thing. specifically when you see two sets of them about 20 feet apart on the road. Might those be for checking speed? ---
**Author:** ![IcicleFuzz](https://avatars.discourse-cdn.com/v4/letter/i/ba8739/32.png) [@IcicleFuzz](https://boards.straightdope.com/u/IcicleFuzz)\ **Post date:** [August 2, 2003, 5:17am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/4 "2003-08-02T05:17:14Z")
My friends father, who is a council worker, tells me that they measure speed and weight of the vehicles, as well as the time that it passed over the cables. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [August 2, 2003, 10:13am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/5 "2003-08-02T10:13:36Z")
> [@](#): > > I’ve wondered the same thing. specifically when you see two sets of them about 20 feet apart on the road. Might those be for checking speed? They could be for speed or they could be checking volumn on 2 lanes. One rubber hose will only cross 1 lane while the 2nd will cross 2. subtracting the ‘hits’ of #2 from from #1 will give the cars in lane 2 (and lane one will be the hits on #1) ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [August 2, 2003, 1:23pm UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/6 "2003-08-02T13:23:48Z")
> [@](#): > > \*Originally posted by Berkut \* > \*\*It’s a similiar setup to a service station bell hose. \*\* I just realized as I was reading this that there is a whole generation of people that has no idea what a service station bell hose is. ---
**Author:** ![USCDiver](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/uscdiver/32/354_2.png) [@USCDiver](https://boards.straightdope.com/u/USCDiver)\ **Post date:** [August 2, 2003, 4:07pm UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/7 "2003-08-02T16:07:47Z")
Nah, they got 'em at the Jiffy Lube. ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [August 2, 2003, 6:14pm UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/8 "2003-08-02T18:14:04Z")
Here’s an interesting bit of information: Bill Gates’ first entry into the entrepreneurial world was a company he started called “Traff-O-Data”. They made and sold the aforementioned devices. ---
**Author:** ![Ms2001](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ms2001/32/18463_2.png) [@Ms2001](https://boards.straightdope.com/u/Ms2001)\ **Post date:** [August 2, 2003, 11:47pm UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/9 "2003-08-02T23:47:48Z")
How can they measure speed of the vehicles… won’t the varying length of cars mess up the data? A car with 10 feet between the axles going 60 MPH will look the same to the machine as a 5 foot car (or motorcycle) going 30 MPH. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 3, 2003, 12:10am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/10 "2003-08-03T00:10:02Z")
The ones that (I assume) are measuring speed have a pair of parallel cables spaced about a foot apart (at least I’ve seen a lot like that around here lately). I plan to learn to ride a unicycle, just so I can mess up the data. ---
**Author:** ![Ashkicker](https://avatars.discourse-cdn.com/v4/letter/a/4bbf92/32.png) [@Ashkicker](https://boards.straightdope.com/u/Ashkicker)\ **Post date:** [August 3, 2003, 5:29am UTC](https://boards.straightdope.com/t/what-are-those-cables-you-see-in-the-road/192502/11 "2003-08-03T05:29:21Z")
Hey I like that idea **Mangetout**. I never was much for conformity. I wonder if you just stopped the car right on top of one and went back and forth over and over it. Yeah, I like that idea. Take that and put it in your data records. 🙂