[paul.fr]

There is a stretch of motorway near me that has 6 camera's over 3 miles, according to the sinage, they measure your average speed. So how does that work?

Do they somehow assign a number to a car? How do they keep track of all individual cars over a 3 mile distance?

[techmind]

It's done using Automatic Number Plate Recognition  (ANPR)

[another_someone]

They do not keep track of every car that passes through.

As Techmind suggests, they take an image of the cars (as many cars as they can), and then try and identify the number plate, and read the numbers (sometimes getting it wrong - particularly if they are foreign number plates).  This is technology that was originally used to protect London from car bombs at the time the IRA were bombing London.  It is also used to check for cars in congestion charge zones, to make sure that any car entering the zone has paid the congestion charge they should have paid.

It then, as I guess you have already surmised, take an image of your car further down the road, check the number plate again, and check it against the number plates they read before.

Each individual camera checks only one lane of traffic, but I understand the cameras only work in pairs, so that if you have three lanes being checked at one point, each camera in the first set is paired with a camera in the second set, and if the camera that picks you up in the first set is from a different pair than that which picks you up in the second set, then it wont be able to calculate your speed.  Since this is only a limitation in the software, I am surprised that this situation exists, or at least that if it has not been overcome yet, I would expect it would be overcome soon.

[techmind]

I understand Trafficmaster use a similar system to establish average traffic-flow rates and identify traffic-jams, slow-moving traffic and other effects automatically, to feed into their paid-for travel-information service (can be used to augment sat-nav to avoid heavy traffic etc using real-time information). They have (commonly dark-blue painted) cameras on major trunk roads and motorways.
Trafficmaster have been in this business in the UK for over 12 years in the UK.

[another_someone]

I understand Trafficmaster use a similar system to establish average traffic-flow rates and identify traffic-jams, slow-moving traffic and other effects automatically, to feed into their paid-for travel-information service (can be used to augment sat-nav to avoid heavy traffic etc using real-time information). They have (commonly dark-blue painted) cameras on major trunk roads and motorways.
Trafficmaster have been in this business in the UK for over 12 years in the UK.

Yes, but as I understand it, trafficmaster don't read the entire number plate, only 3 digits, or some such; and if trafficmaster gets it a little wrong, nobody is going to find an erroneous speeding ticket landing on their doormat, so it can afford to make far more mistakes, and assume that it will have little impact on the overall average answers it needs.

Otherwise, I do agree that the underlying principle is the same.

[lyner]

It is amazing that, just as we are able to install extensive number plate recognition systems, they have eased (or seem to have) the restrictions on typeface and size of number plates. You can see 'all sorts' these days.
What is needed is 'spy in the cab'. Sat Nav is very cheap and all that is needed is a WI FI, style interrogation system. The boxes could be tamper proof - mounted externally - and tell the authorities exactly what we have all been doing. (More like what YOU have all been doing)
A big red light could come on when you are speeding, perhaps, visible to everyone.
The only problem would be in handling the mass of information that would come out of such a system.
How do we feel about speed restrictions, anyway? Personally, I don't find it's the speed restriction that is the major contributor in restricting my average speed; it's all the rest of you, getting in the way. (And the old dears who want to do 40mph when they could do 60.)

[another_someone]

How do we feel about speed restrictions, anyway? Personally, I don't find it's the speed restriction that is the major contributor in restricting my average speed; it's all the rest of you, getting in the way. (And the old dears who want to do 40mph when they could do 60.)

The major limiting factor in journey time, at least is anything but long motorway journey's, is time spent standing still (e.g. at traffic light, and road junctions).

Overall, I think the speed limits in place are largely discredited (i.e. almost everyone ignores some percentage of them, and so bringing the entire system into disrepute), but it is not politically acceptable to make them more realistic and appropriate (given that they were designed for road conditions that existed half a lifetime ago).