OT: Traffic monitoring cables across road

[Caecilius]

This is a bit off-topic, but does anyone know what the traffic monitoring devices that consist of two parallel cables running across the road at right angles to the direction of traffic are called and what they are used for?

They seem to have appeared recently on the roads in my area (Medway in Kent), and I recall seeing them last year as well - perhaps they are seasonal?

I presume that they are placed by the council, and record the quantity and speed of traffic (or, more specifically of pairs of wheels). However, does anyone know what the data is used for? Is it used to decide where to place speed cameras or some other neferious purpose?

[Newshound]

I read an article on these some time ago which said they were basically used for traffic surveys. If you think about it, you will see that with two pipes "they" can work out the speed, wheelbase, and number of axles for each vehicle. So they can more or less differentiate between motorbikes, cars, buses, and lorries of different sizes. (If two vehicles pass in opposite directions at the same time the software drops the data in the "too difficult" box).

[R.P.McMurphy]

Inductive loops, yep they're used for counting cars.

Steve

[Andy Hall]

It's certainly nefarious.

There will be the council traffic department and then an outside firm of consultants to do the work, thus doubling the cost.

The rest is detail.

[Andrew Mawson]

(Medway in

quantity and

However,

IME they are usually pneumatic and not inductive loops. The permanent buried ones for trafic lights are inductive, wheras the temporary surface mounted ones for trafic census purposes are a rubber tube, with a metal plug at the far end and a pressure sensor within the instrument, which is usually padlocked to a lamp post.

AWEM

[fred]

In article , Newshound writes

That's a pretty good summary I reckon, only thing to add is that they used to be used to collect data when a complaint was made of excessive speeding on a stretch of road, the stats received could be used to justify a speed trap on that stretch.

[Newshound]

NO! The consultants tell the council what to do, it's the contractors who do the work (tripling the cost....)

(a consultant)

[Andy Burns]

Some of the newer ones I've seen are a metal box big enough for e.g. a couple of car batteries with a 15' post coming out of it, and a camera on top, usually the box chained around the base of a lamp-post and the camera-post is duct taped to the lamp-post higher up for support, presumably there is a video or hard disk recorder inside, they only seem to stay at any one site for a day.

[R.P.McMurphy]

yep your right, if they are imbedded they are likely inductive loops, on the surface they are the pneumatic type of counter.

Steve

Steve

[The Medway Handyman]

Where are they?

Knowing Medway Council its bound to cost us money :-(

[Owain]

google Lochwynd - they do this sort of thing

Owain

[Ian White]

And the cut-outs in the road are... loop shaped. Although buried loops are intended to be permanent installations, they aren't all in continuous use, as the sensor equipment may well be moved around from site to site.

Loop sensors are basically metal detectors, but with two loops per lane and a bit of clever computing they can also work out speed, length, gap between following vehicles, and even classify vehicles into cars, vans, rigid lorries, artics etc.

Those are single-ended tubes, as someone described. They are only ever temporary, as they wouldn't survive as a permanent installation. Usually they are used by county traffic departments to answer specific local questions that aren't covered by their long-term monitoring schemes.

Counties and the Highways Agency have been collecting traffic data for years, and there has been an upsurge in activity because of maintenance contracts that are formula-funded according to the volume, speed and types of traffic.

One thing all of these loop and tube sensors have in common is that they are strictly for collecting traffic data. They aren't interested in individuals. Above all, this type of equipment is categorically not capable of 'prosecution quality' speed measurements.

[Lurch]

On Mon, 18 Jun 2007 21:22:00 +0100, "Newshound" mused:

How do they do that then? How does it know the difference between 2 twin axle vehicles and one 4 axle vehicle? Ho does it know when a 3 axle vehicle ends and a 2 axle one starts?

Why are there devices with 1 or 2 tubes?

[nightjar]

They are normally part of a traffic survey, aimed at deciding how heavy the traffic is in relation to the road designed capacity. The usual process is to start with a manual count, which involves several people sitting in cars at the same spot for a whole day, operating a cumulative counter for every vehicle type that passes the bit of road they are responsible for - there will usually be at least one car for each direction of travel and several roads around a junction may be surveyed at the same time. That gives data on the type of traffic, from which axle numbers are derived. The rubber tubes then operate pneumatic counters, which count the axles and direction of travel over a much longer period. They can also check traffic speed at different times, to determine when the road is congested. Combining the data gives a pattern of road use and is used to help plan road development in the area. A traffic census may also form part of the survey, to decide whether the traffic is primarily local or is passing through.

Colin Bignell

[The Natural Philosopher]

They are used to justify putting speed humps and chicanes in. Always slow down for em or you will end up with a nightmare of road furniture.

[The Natural Philosopher]

The distance between the tubes is small enough so that long duration pluses due to one wheel and then the next wheel are discounted.

2-tube devices monitor speed as well as traffic density.

Cutting the tubes or moving them further apart will render them irrelevant.

[Bob Eager]

Apart from the pairs of pneumatic tubes that used to be (may still be) embedded in roads as traffic light sensors...

[Ian White]

Think about the information you get, and what you could then do with it.

One pneumatic tube can only count wheel crossings, and you don't even know which direction the vehicles are travelling. Two tubes stretched across a two-lane road will give direction and a rough indication of speed, though the pulses of air pressure only give poor timing information. Rather than precise wheel crossings, loops detect generalised lumps of metal overhead, but overall they tend to give more reliable results.

With some real-time computing capacity right there at the roadside, they can sweat a surprising amount of information out of the raw sensor signals. All the differences that you can see with your own eyes have some equivalent in terms of timings and signal levels.

Differentiation between types of vehicles is mostly based on timing. For example, in freely flowing traffic the timing patterns for a 4-axle vehicle and two 2-axle vehicles are very different (or if vehicles really are crawling nose-to-tail, they will know that from the speed information). Also, wheels that belong to the same vehicle will all have the same speed (within a few percent) and there are only a few common patterns of wheelbase and length. With loop sensors there is additional information from the signal level: a long artic will always have some metal overhead, but in gaps between vehicles the signal drops right out.

Put it all together, and they can infer a lot of information from very simple sensors. It isn't 100% reliable, but close enough for census work.

[John]

Really interesting web site. Thanks

[Ian White]

Those are quite rare now. When they wear out, they are almost invariably replaced by loops, which are the right kind of sensor for traffic lights because they can also detect stationary vehicles.