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

B0bbyEwing

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....?

400ixl

That will entirely depend on the position of the camera and the road. Cameras can be at different heights and angles. 

What is the reason behind the question as that may give some clearer answers.

Car_54

Are you sure about the camera ‘facing you’? AFAIK they all face the same way.

Two cameras, set a known distance apart, capture you, and it’s easy to get the average speed from the time difference. Assuming they’re identical cameras, the ‘capture point’ (x metres from the camera) should also be identical. Any practical differences of a few feet in setup will be negligible compared to the distance between the cameras, and indeed in relation to the 110% + 2 enforcement threshold.

user1977

Why are you asking? I can't see it matters exactly where they detect you, as the point is they're measuring your average speed over the much greater distance between the cameras. And they will have measured that distance, for the benefit of anybody who tried to pick holes in tickets.

daveyjp

Tens of metres as they rely on illumination to read plates in the dark,

molerat

The capture distance from the actual camera ain't gonna make that much difference on the 31 mile single enforceable section of the A9 !

born_again

Look at the angle of the camera, that will give you a idea of how far away it will measure from.

But why just A roads? M Ways are just the same.

LunaLater

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....?

They don’t tend to face towards you, they face away from you. Given the angle, they look to be pointing at the road about 100m past them.

It’s completely arbitrary of course, as long as they are all pointing the same distance ahead.

Why do you ask?

Okell

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....?

Why are you interested in their "capture zone" and their "range"?  Neither really means anything with average speed cameras.

You have two precisely located points - Point A and Point B - on the road which are a known distance apart.  I don't know what the minimum* distance between them must be, but they could be several miles apart.

Camera A simply records your car passing Point A at a precise time, and Camera B records it passing Point B at a precise time.  Because the distance between Points A and B is known to a high level of accuracy, your average speed is easily - and accurately - calculated.  The greater the distance between A and B the greater the accuracy of the calculation.

The distance between Points A and B will vary from one set of cameras to another.  So "capture zone" and "range" are irrelevant.

Perhaps I've misunderstood but you seem to be thinking you have two cameras at different locations which take instantaneous speed readings and then take the average of the two readings.  If you are, that isn't how they work.



*I'd have thought it really needed to be > one mile but it might be less than that.

facade

It seems to be a closely guarded secret.

The measurement point must be at a specific point in the frame, and any one with common sense would have put calibration markers out at the precise start and end points to make it easy to set the cameras up, yet there don't seem to be any.

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). I suppose over 10 miles 50 yards or so won't be outside the claimed accuracy (about.0.3%)

They must be a nightmare to position and adjust!

They could always point them down at 15 degrees say, the same height from and parallel to the road and then time from when the width of the vehicle is x pixels on each camera and use the distance between poles.

A white line across the road would make it a lot easier- I wonder why they don't use them?

(Then they could print out the timestamped frames where your vehicle crossed the lines along with the distance between them and there would be no room for argument that the distance wasn't exact)

SiliconChip

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?