Cables on the road?

[Iain]

I've noticed these things a lot in my local area over the past few years and have gone over several of them on the route to work. Basically it's two cables going across the road, tied up to a flat box thing at either side on the pavement. Are they perhaps something to do with speed cameras (such as surveying how much traffic passes those points), as they are all in places where the 'Safety Camera' team can be found?

[Mark]

They can be for several reasons. They can be used to monitor traffic flow, bascially to see how busy a road is in order to see if improvements (hahaaha!) are necessary, two cables are needed to tell the difference between traffic in each direction.

As you suspect they can also monitor the speed of the traffic passing over them, in order to see if they could make money out of a speed camera (nb. they can't make money out of speed cameras any more anyway, they've changed the rules now)

There have been places where camera vans 'plug & play', i.e. they hook themselves up to these cables in the road and it pings them whenever there is a car going too fast, then they nab them on their camera, but from what you describe it doesn't look like this type.

[Dave B]

Whenever they've appeared round our way, it's a surefire bet that some absurd "traffic calming" measure will appear a few months later :\

[schmenke]

I've only ever seen them as a vehicle-counting device.

To use them to accurately measure a vehicle's speed wouldn't the distance between the axles have to be known?

[Hazell B]

They take an average Schmenke, then work out if cameras, speed bumps or whatever would be good there or not.

I heard a story about somebody cutting a road cable (he thought it was to catch speeders) and getting major shocked, only to fall under a lorry's wheels. No idea how true it is, but the end of the story made me laugh - the cable was for a short term building site's power!

[Mark]

I've only ever seen them as a vehicle-counting device.

To use them to accurately measure a vehicle's speed wouldn't the distance between the axles have to be known?

No, why would it? You take the time between the first cable being hit and the time between the second cable being hit. They are placed close enough together so for most cars the front wheels will be over the second cable before the back wheels hit the first one.

If there it gets confused or there is traffic going the opposite way it'll just ignore those results.

[schmenke]

Ah, are there two cables?
I've only ever seen one stretched across the road.

[Dave B]

You do occasionally get one cable which can act as a simple counter, but takes no account of speed or direction.

[Hazell B]

No offense, but two cables can't possibly be relied on for accurate speed data. Cars stop, get over taken, have double axels, etc. Without cameras it just cannot work. You'd never know which set of wheels belonged to which vehicle by the time ten had gone over!

[Dave B]

True, but they're not relied upon for prosecutions, merely for assessing either traffic flow or the need for calming. For that purpose they're accurate enough.

[Iain]

It does make sense that they're for assessing traffic flow. Putting them at all the points where the mobile safety cameras are probably helps with their figure for justifying their existence.