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

facade

Car_54 said:

Nearlyold said:

HHarry said:

I think the point the OP makes is that if a forward facing camera is recording your speed half a mile away then you could theoretically speed up after that point and “safely” be doing more than the limit as you pass under the camera.

Equally if a rear facing camera records half a mile away then you need to maintain you speed as you pass under the camera and for a further half mile.

 If you know the detection points you can game the system by increasing / reducing your speed earlier or later rather than at the speed limit signs.  But the nett benefit is going to be negligible and the proper answer is of course “just follow the speed limit signs”.

But if there's a third camera wouldn’t you then have to slow down to below the speed limit  to reduce your average speed otherwise the third camera will  pick you up as speeding. As you say seems an awful lot of bother for very little gain.

If there is a third (or fourth, or fifth) average speed camera, then the speeds are measured between the adjacent pairs, not over the whole section.

The cameras work on the time taken between them. If the time is too short, you are speeding.

Measuring between anything other than adjacent cameras (assuming there are no dummies) would offer no advantage, and could be counter productive.

If the cameras are A B C  then if your time between A&B is too short but you slow down significantly and increase your time for BtoC, then the overall time for  AtoC could be below the limit. (You could even turn off before you get to C)

On the other hand, if your overall time is too short for AtoC then it must have been too short for AtoB or BtoC or both anyway..

Nearlyold

Car-54 Then what I said would still apply (assume you mean from 1-2 then 2-3 then 3-4  4 -5 etc). The only stretch where you could game the cameras and not get caught for exceeding the legal average speed would be  after you'd passed the pick up point of the last camera

Car_54

Nearlyold said:

Car-54 Then what I said would still apply (assume you mean from 1-2 then 2-3 then 3-4  4 -5 etc). The only stretch where you could game the cameras and not get caught for exceeding the legal average speed would be  after you'd passed the pick up point of the last camera

In other words, you can't be caught by average speed cameras except in an average speed camera zone?

born_again

This reminds me of the Cat & Fiddle road.
Bikers soon learned that they only way to enjoy this road to the full was to stap en-route for so long & then finish it off. Other option was a fine 🤷‍♀️

Petriix

Car_54 said:

Nearlyold said:

HHarry said:

I think the point the OP makes is that if a forward facing camera is recording your speed half a mile away then you could theoretically speed up after that point and “safely” be doing more than the limit as you pass under the camera.

Equally if a rear facing camera records half a mile away then you need to maintain you speed as you pass under the camera and for a further half mile.

 If you know the detection points you can game the system by increasing / reducing your speed earlier or later rather than at the speed limit signs.  But the nett benefit is going to be negligible and the proper answer is of course “just follow the speed limit signs”.

But if there's a third camera wouldn’t you then have to slow down to below the speed limit  to reduce your average speed otherwise the third camera will  pick you up as speeding. As you say seems an awful lot of bother for very little gain.

If there is a third (or fourth, or fifth) average speed camera, then the speeds are measured between the adjacent pairs, not over the whole section.

I believe more recent average speed checks can use combinations of zones; if, for example, you are obscured between two HGVs at camera B, you could still be caught if your total time between cameras A and C was too short. 

molerat

Petriix said:

Car_54 said:

Nearlyold said:

HHarry said:

I think the point the OP makes is that if a forward facing camera is recording your speed half a mile away then you could theoretically speed up after that point and “safely” be doing more than the limit as you pass under the camera.

Equally if a rear facing camera records half a mile away then you need to maintain you speed as you pass under the camera and for a further half mile.

 If you know the detection points you can game the system by increasing / reducing your speed earlier or later rather than at the speed limit signs.  But the nett benefit is going to be negligible and the proper answer is of course “just follow the speed limit signs”.

But if there's a third camera wouldn’t you then have to slow down to below the speed limit  to reduce your average speed otherwise the third camera will  pick you up as speeding. As you say seems an awful lot of bother for very little gain.

If there is a third (or fourth, or fifth) average speed camera, then the speeds are measured between the adjacent pairs, not over the whole section.

I believe more recent average speed checks can use combinations of zones; if, for example, you are obscured between two HGVs at camera B, you could still be caught if your total time between cameras A and C was too short. 

The A9 apparently uses the average speed between any 2 cameras within each of the 19 northbound and 18 southbound designated zones over the 136 mile monitored length, the longest individual zone being 31 miles.

It is obvious where you are being captured on many cameras on that road by the lamp post at the side of the road with the infra red illuminators.

ontheroad1970

As @Okell says, zone is irrelevant.  They aren't recording instantaneous speed.  They are recording when a car passes two different cameras on a simple "Speed= Time/Distance.  They may or may not be consecutive cameras, or they may be different camera pairs.

Okell

HHarry said:

I think the point the OP makes is that if a forward facing camera is recording your speed half a mile away then you could theoretically speed up after that point and “safely” be doing more than the limit as you pass under the camera.

Equally if a rear facing camera records half a mile away then you need to maintain you speed as you pass under the camera and for a further half mile...   

Sorry - but you seem to be demonstrating the same lack of understanding as the OP.

Average speed cameras are not "recording" your speed from half a mile away.  

What they are doing is recording the time when you pass points A and B where the start and end cameras are  situated.  Because the distance between points A and B is known to a high degree of accuracy, an accurate average speed between points A and B can easily be calculated. 

Yes - average speed cameras can be "gamed", but that has absolutely nothing to do with them detecting you from half a mile or a mile away - because they can't do that!

CliveOfIndia

molerat said:

The A9 apparently uses the average speed between any 2 cameras within each of the 19 northbound and 18 southbound designated zones over the 136 mile monitored length, the longest individual zone being 31 miles.

What puzzles me on the A9 is where it switches from single-carriageway to dual-carriageway and back again, which it does many times.  On some sections there's a camera at the start and end of each "change", which is fair enough.  But there are some stretches where a single pair of cameras span both types of road - how does that work?  So you do 60mph on the single-carriageway section and 70mph on the dual-carriageway section - you're average speed is going to be 65mph or whatever.  Does that mean you'll be fined for doing more than 60mph on the single-carriageway stretch, when in fact you weren't?

Although I guess doing 65/66 in a 60 zone is borderline for a fine anyway, so do they just have a built-in tolerance that covers that eventuality?  And if you were really doing 67 in the 60 and 79 in the 70 then you'd be over the tolerance?

Mildly_Miffed

Car_54 said:

Nearlyold said:

But if there's a third camera wouldn’t you then have to slow down to below the speed limit  to reduce your average speed otherwise the third camera will  pick you up as speeding. As you say seems an awful lot of bother for very little gain.

If there is a third (or fourth, or fifth) average speed camera, then the speeds are measured between the adjacent pairs, not over the whole section.

They're both.

If you pass A, but B misses you (your number plate is shielded from the camera's view by a truck or whatever), then C spots you - the average speed between A and C is still calculated, not just A-B then B-C.

Back in the early days of motorway average speed cameras, urban legend was that they couldn't track you between lanes.
So if you passed gantry A in lane 3, then B in lane 2 - no A-B average. But pass C in either lane 2 or 3, there was an average (B2-C2 or A3-C3)

Ultimately, with average speed cameras, there's simply no point in trying to game them. Just accept that they will catch you if you speed through the section., and chill out a bit.

If the average cameras cover a five mile section, then by doing 60 rather than 50, you'd only save one minute. Is it really worth the risk?