what are the two cables that lay across the road do, and how do they work?

[-scrumpy-]

You're drivnig along the road, and you see a small black box on the verge, and two cables coming out of it that lay across the road, parallel to each other. Do they measure speed, or do they count traffic? Whichever, however do they work? How does it know I'm in a car or a 4 axle lorry?

thanks!

[ugly_bob]

they measure speed and traffic volume.
They can tell the difference between vehicles by their wheelbase length

[Catso]

I thought they just counted axles going over them. It'd be tricky to tell the difference between a 4 wheel tractor pulling a 4 wheel trailer and two cars.

If this is so, I'd imagine the user just multiplies the count by know values to get a value for the number of vehicles. Which would only be an approximate value, but good enough for most purposes.

I believe they work by sensing air pressure changes in a sealed tube.

[ugly_bob]

you might be right catso, but it would be relatively simple to tell the difference between an artic and 2 cars because of wheel spacing from speed (for the double cabled ones anyway).
Would give a slightly more representative view than just number of axles, although, in practice not many trailers have the same wheelbase as a car.

[Catso]

I disagree. A Mini followed by a Smart Car would pretty much exactly match an artic. Or a following motorbike would be even better.

I look forward to a highways employee telling us how it's really done...

[heathfield]

It's traffic volume they're measuring, by number of axles counted. That's what wears the road out. They don't measure speed in themselves, (otherwise a close-wheeled trailer might seem to be doing 150 MPH!)
Catso's right. They're a sealed rubber tube, and when a wheel passes over it, air pressure activates a pneumatic counter.

[ugly_bob]

I'm sure the single lines measure volume, but the double ones measure speed also - otherwise why would they bother with 2?

[munchhausen]

it's impossible to measure speed accurately unless you know the exact wheel base of the vehicle concerned. They do however record an average road speed as well as traffic volume. This enables town planners to determne the level of traffic calming measures necessary on a stretch of road.

[ugly_bob]

that's why they have 2 cables - they know the distance between them, and can calculate the speed from this independent of the wheelbase of the vehicle.

[munchhausen]

That's only true if you know the wheel size. a larger wheel moves across the ground further in one rotation than a small one. therefore unless you know the wheel size an accurate speed assesement can not be made because at the same speed, over the same distance a large wheel will appear to be travelling faster than a small wheel simply because it covers more ground more quickly.

[Space]

no, munchhausen, the cables don't measure the rotation speed of the wheels.
They can only measure the car speed, regardless of wheel size.

[munchhausen]

SPACE - I didn't say they did measure rotation. If you re-read my post you'll understand WHY they can't accurately measure speed. Think about the principle of gearing then you might understand why wheel size is relevant.

[sddsddean]

Heathfield is spot on. They are purely counting axles, as this is how the road surface is designed. The reason for having 2 'cables' is to show which direction has a higher traffic volume. Some detectors record the figures' over time' instead of just numbers, so the effects of things like the rush hour can be shown. On most new dual carriageways you will see loop detectors built into the road surface with a little box by the side of the road, normally on a 2 foot high pole. This is a fixed axle counting point, but is only used when the 'gubbins' is placed in the box. By doing counts at different times of the year, seasonal changes can be allowed for eg tourist traffic in the West Country in Summer.

[Loosehead]

Oh dear munchy, doesnt matter what size the wheel is for measuring purely speed. Bob is correct. Say the cables are one metre apart then when the wheel hit's the first the centre of the wheel is directly above the cable. Doesn't matter whether its one inch or one yard high. Then when it passes the second, 1 metre away the centre of the wheel has travelled 1 metre. Time it and hey presto!

[Space]

munchhausen, you don't understand. the "principle of gearing" , as you say, is about converting a rotational speed to a linear speed.
In this case, the system just measures a linear speed already. No conversion is needed, no rotational speed involved, no wheel size involved.

[williaid]

there is no way they can measure speed... is a mini doing 30mph really an lorry doing 80 mph?

all they can do is count axles going over them

[munchhausen]

Chalk a mark at the bottom of a wheel. roll the it around until the mark hits the ground again. A large wheel will have moved further than a small wheel. When the only thing being recorded is the time it takes to move between two fixed points the small wheel has to move much faster to cover the same distance in the same time as the larger wheel. Not difficult really!

[gammaray]

Quite right Munchhausen!

[Loosehead]

well you can take horse to water.......

[Ralph]

munchhausen, consider this:
You are zipping along the road on your hoverboard which normally has nothing touching the road. Today, however, your backpack is dragging along the road. You approach two of these rubber tubes across the road that are one metre apart.
Your bag hits the first one and then 0.0746 seconds later, your bag hits the second one. The little box can work out that your hoverboard was doing 30mph without knowing anything about rotation at all (there wasn't any).

It doesn't matter about rotation at all, a Penny Farthing bicycle with wildly differing wheel diameters (and hence wheel rotation speeds) will register the same speed from both wheels. If they're not going the same speed, the rider's in big trouble, maybe that's why they don't use them anymore?