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

daveyjp

CliveOfIndia said:

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?

No puzzle, you need to forget speed and consider time and distance, as this is what is recorded to calculate an average speed.

 If a stretch is 20 miles long and it starts at 30, goes to 40, then to 70, back to 60, if you drive exactly at the limits it will take you a certain amount of time to cover the 20 miles.  Say 30 minutes.  This is the minimum amount of time, so an average speed of 40 mph.

The 20 mile dostance may also be broken  down into sub sections to calculate intermediate average speeds.

If a driver covers the 20 miles in 20 minutes it substantially less than the minimum time so they have been speeding as 20 miles in 20 minutes is an average of 60mph.  Your absolute speeds at any one time over the distance doesn't matter.

You do however need to be wary that average speed camera sections may also be covered by instant speed cameras!

CliveOfIndia

daveyjp said:

Your absolute speeds at any one time over the distance doesn't matter.

Thanks for that - makes sense I suppose.

Not that it matters as such, I'm always pretty careful about such things (not saying I've never ever broken the limit in my entire life, but I do try to be careful!).  I was more just curious about how it worked in that situation 

Krakkkers

So if the cameras are forward facing then they do not pick up motorcycles?

facade

Krakkkers said:

So if the cameras are forward facing then they do not pick up motorcycles?

Correct.

And also probably not the "fashionable" cars with the front numberplate on the dashboard either, but I wouldn't want to bet £100 on it.

Grey_Critic

Personally I am more concerned that the speed limits are clearly signed/posted, I am regularly overttaken in areas where there are cameras and the immediate braking when they realise there is a reason for me doing a particular speed not just dawdling.

Jenni_D

molerat said:

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.

The A9 also has cameras on the off-ramps, so you can't speed after the "last" camera thinking there won't be another before you turn off.