Average speed cameras on A-roads - how do they work?

Okell

facade said:

... The minimum distance between cameras is 200m I believe, so a 20m total error in the measurement points leaves you with 2mph tolerance on speed, if they follow enforcement guidelines (the 10% is used up by the camera)...

If the claimed accuracy is 0.3% then 50 yards over 10 miles is just within that.  (0.284%)

But you've lost me with the bit in bold(?)  

The minimum distance might well be 200m, but why are you introducing the idea of a 20m (or 10%) error into the distance measurement?

I doubt very much that whichever agency is responsible for setting up these cameras could be so incompetent as to introduce a 10% error in the distance measurement between cameras, no matter how far apart they are.

I'll admit I don't know how Points A and B are identified for these purposes, but I'd be surprised if the distance between them had not been properly surveyed before the cameras were installed.  Certainly over 200m I wouldn't expect an error greater than around 1m (0.5%).

But then again...

[Edit:  I've just realised you've chosen 20m or 10% because of the "10% + 2mph" enforcement guidelines.  As I said - I don't think it at all likely that there would be a 10% error in the distance measurement.

But as I also said: "... then again..."]

shinytop

B0bbyEwing said:

This will test anyone who responds to just a thread title vs reading the message body

With the static cameras with the white lines painted on the road you knew exactly where the 'capture zone' was. 

But you enter one of these average zones after one camera. You're driving towards a camera that's facing you - at what point does it detect you, as in what is their range?

"Just do the speed limit and you'll have nothing to worry about". 
For those who were going to say that I've saved you the effort  
So ... the range....?

I'll save you the effort of reading all the other replies and say the same as everybody else.

I don't know.

Aretnap

SiliconChip said:

B0bbyEwing said:

With the static cameras with the white lines painted on the road you knew exactly where the 'capture zone' was. 

Isn't it true that the white lines are now irrelevant as static cameras don't work in the same way as they used to?

Depends on the type of camera but if you mean the classic "dragon's teeth" in front of a Gatso-type they're still relevant, but they're not essential and never have been. The camera has to generate two independant measurements of speed. The primary measurement comes from the radar that triggers the camera to take two photographs and the secondary measurement is the distance that the car has travelled in the half-second or so between photographs. The easy way to measure the distance travelled is to count the number of white lines that the car has crossed, but if the lines are faded or otherwise not visible there are approved techniques based on photogrammetry (ie the change of size and position of the car in the images) which can be used instead.

chrisw

I don't think I've ever seen average speed cameras other than on the motorway. Are they just used on higher speed A roads and I presume there are signs saying you are now entering an average speed zone?

user1977

chrisw said:

I don't think I've ever seen average speed cameras other than on the motorway. Are they just used on higher speed A roads and I presume there are signs saying you are now entering an average speed zone?

They're all signed, permanent ones I'm aware of are the A55 (which is quasi-motorway), and A9 (as mentioned above), and A77 (the latter both being largely single-carriageway). I'm sure there'll be plenty of other examples. Also seen temporary ones on A roads during longer term roadworks.

Database of all sorts of speed cameras here, if you click through you can see from the icons which are average cameras: https://www.speedcamerasuk.com/locationsdatabasetext.htm

daveyjp

A9 has plenty and has made the road far safer.

Leeds Council are currently installing average cameras along a couple of stretches of the ring road.

Aretnap

chrisw said:

I don't think I've ever seen average speed cameras other than on the motorway. Are they just used on higher speed A roads and I presume there are signs saying you are now entering an average speed zone?

Near me I can think of one, on a 50 mph single carriageway and a 40mph single carriageway (fenland road next to a river with no crash barriers, speed limit in force to stop local boy racers ending up underwater). Have heard if them being used on urban roads but average urban speeds are usually well below 30 or even 20moh anyway so it would be a fairly niche road where they were useful.

Generally they are a good thing. As usual 10%+2 tolerance applies, to be caught by an average speed camera you have to be going quite significantly over the limit for quite a long way. Your not going to be penalised for drifting slightly over the limit for a brief moment as you could be with a fixed/mobile camera. In this sense they are "fairer on the motorist", if you subscribe to that sort of thinking.

facade

There is, of course, no reason why they can't site a mobile camera within an average speed zone, to catch the people who (as advised by Proper Top Gear) pull over for 5 minutes or whatever just past the first camera so they can drive at 180 through the rest of the zone.

Car_54

chrisw said:

I don't think I've ever seen average speed cameras other than on the motorway. Are they just used on higher speed A roads and I presume there are signs saying you are now entering an average speed zone?

You presume wrongly. They're often signed, but it's not required legally.

Mildly_Miffed

They work by ANPR.

They log your registration and timestamp on passing each camera, and compare the timestamps over the known distance. If your average speed is above the limit (plus threshold), then it enters the enforcement process, and an S172 notification is sent to the keeper.

60mph is just under 27 metres per second - each kilometre takes just over 37 seconds.
Prosecution is vanishingly likely to happen below 68mph, a bit over 30m/sec - one km in just under 33 seconds.
The length of a short car every second, or four seconds difference per kilometre.

You can easily see that the difference of a fraction of a second in precisely *where* your registration is timestamped is irrelevant over a non-trivial distance.

That's why they use average speed cameras - because of the kind of driver who slows down to pass a camera than speeds up again...