Average Speed Camera Placement

[Maki65.]

I was driving near where I live today and it is an average speed limit road. However, I noticed one camera I passed was in a 50 but the second was just in a 30, by around 50m or so. How do they work out that average? I did have to slow down a little faster for the 30 than I normally would but I was at 30 entering. I am just curious how the calcultae something like that distance and speed.

[Mandat]

Maki65. said:

I was driving near where I live today and it is an average speed limit road. However, I noticed one camera I passed was in a 50 but the second was just in a 30, by around 50m or so. How do they work out that average? I did have to slow down a little faster for the 30 than I normally would but I was at 30 entering. I am just curious how the calcultae something like that distance and speed.

At 50mph, it takes 2.2 seconds to travel 50m, so the 50m overlap at the limit change won't make a significant difference to the speed calculation, particularly since the prosecution threshold (traditionally 10% + 2) will provide some leeway & a margin of error anyway.

[oyster]

Mandat said:

Maki65. said:

I was driving near where I live today and it is an average speed limit road. However, I noticed one camera I passed was in a 50 but the second was just in a 30, by around 50m or so. How do they work out that average? I did have to slow down a little faster for the 30 than I normally would but I was at 30 entering. I am just curious how the calcultae something like that distance and speed.

At 50mph, it takes 2.2 seconds to travel 50m, so the 50m overlap at the limit change won't make a significant difference to the speed calculation, particularly since the prosecution threshold (traditionally 10% + 2) will provide some leeway & a margin of error anyway.

It’s a moot point because, if what the OP describes is correct, then no-one can be prosecuted if average speed limit enforcement is carried out across 2 limits.

[Maki65.]

oyster said:

It s a moot point because, if what the OP describes is correct, then no-one can be prosecuted if average speed limit enforcement is carried out across 2 limits.

This is pretty much as I thought. it seem a strange place to put the camera, particularly as there is a 30 mph Gatso a few metres on!

[Super Sonic]

Maki65. said:

This is pretty much as I thought. it seem a strange place to put the camera, particularly as there is a 30 mph Gatso a few metres on!

It does seem strange. Is it facing downstream into the 30 mph zone, or upstream recording when drivers pass the 30 mph sign?

[zbc]

I assumed that average speed cameras were working out your speed over a distance so if they are spaced 100m apart and half of that is a 50mph zone and half is a 30mph zone then your speed should be an average of 40mph and it should take 5.59 seconds to go 100 metres. If the second camera catches you there after only 5 seconds, expect a letter

[Maki65.]

Super Sonic said:

It does seem strange. Is it facing downstream into the 30 mph zone, or upstream recording when drivers pass the 30 mph sign?

It is facing upstream to traffic from the 50. it is as if that is where there measurement for placing the camera ends up without much thought otherwise. As I stated there is a Gatso a few metres further on for the now entered. It dies look like it is measuring the end of the 50.

Edited by Maki65. on Tuesday 7th April 18:29

[Maki65.]

I need to be clearer, it is facing the 50 which is on a slight slope to the 30.

[BertBert]

zbc said:

I assumed that average speed cameras were working out your speed over a distance so if they are spaced 100m apart and half of that is a 50mph zone and half is a 30mph zone then your speed should be an average of 40mph and it should take 5.59 seconds to go 100 metres. If the second camera catches you there after only 5 seconds, expect a letter

For exceeding the 30 mph limit? If you are accused of exceeding the 30 limit, that cannot be proven by the calculation. I doubt you can be accused of "breaking one of the two limits". It could only work if you'd exceeded the 50+10%+1 for the whole section. I suspect the poster who said that made it unenforceable up the thread is very probably correct.

[Panamax]

It's not complicated. If you arrive at the far end earlier than someone who's driven on the limit all the way - you're nicked.

[Panamax]

BertBert said:

I doubt you can be accused of "breaking one of the two limits". It could only work if you'd exceeded the 50+10%+1 for the whole section. I suspect the poster who said that made it unenforceable up the thread is very probably correct.

How much cash would you want to spend engaging lawyers to defend that position?

[2020vision]

You could calculate a weighted average by assuming a maximum of 50 mph in the 50 limit, then work out the weighted average in the 30 limit. Thus resolving the minimum average in the 30 limit used to cause the speed observed for the full length.
Simples!

[speedking31]

zbc said:

I assumed that average speed cameras were working out your speed over a distance so if they are spaced 100m apart and half of that is a 50mph zone and half is a 30mph zone then your speed should be an average of 40mph and it should take 5.59 seconds to go 100 metres. If the second camera catches you there after only 5 seconds, expect a letter

Thank goodness you don't do the sums for National Highways The average isn't 40 mph and the time taken at the limit is 5.97 s. Not much difference practically in this scenario, but significant elsewhere.

For example, if the limit in an average section is 50 mph, and you complete the first half distance at 40 mph, then you can do 70 mph in the second half for an average of 50.9 mph.

Edited by speedking31 on Wednesday 8th April 01:51

[Yellow Lizud]

Just because the post the camera is mounted on is 50m into the 30 mph zone, it doesn't mean that is where the timing is taken from.
The camera is pointing towards the 50 mph zone and it's actually focal point for taking the time is probably some distance from the camera position, maybe 50m.
I'm sure the start and finish points for the average speed will all be in the 50 zone, so where the actual camera is situated is irrelevant.

Or to put it another way, the position of the two timing points has to be where the ANPR can read the number plate. Those two timing points can't be level with the camera or all the camera would see would be the side of the vehicle.

[GasEngineer]

speedking31 said:

hank goodness you don't do the sums for National Highways The average isn't 40 mph and the time taken at the limit is 5.97 s. Not much difference practically in this scenario, but significant elsewhere.

For example, if the limit in an average section is 50 mph, and you complete the first half distance at 40 mph, then you can do 70 mph in the second half for an average of 50.9 mph.

Do the cameras also operate as individual cameras so that if you passed one at 70 it would nick you?

Edited by GasEngineer on Wednesday 8th April 06:56

[E-bmw]

GasEngineer said:

Do the cameras also operate as individual cameras so that if you passed one at 70 it would nick you?

Edited by GasEngineer on Wednesday 8th April 06:56

No, you can't get an average speed from one single point in time on a picture.

[BertBert]

2020vision said:

You could calculate a weighted average by assuming a maximum of 50 mph in the 50 limit, then work out the weighted average in the 30 limit. Thus resolving the minimum average in the 30 limit used to cause the speed observed for the full length.
Simples!

How does assuming 50 in the 50 work? You have no evidence of which limit was broken unless the normal average is greater than 56.

[Maki65.]

The logiacal part of my brain tells me that the camera in the 30 is just measuring the end of the 50.

[zbc]

speedking31 said:

zbc said:

I assumed that average speed cameras were working out your speed over a distance so if they are spaced 100m apart and half of that is a 50mph zone and half is a 30mph zone then your speed should be an average of 40mph and it should take 5.59 seconds to go 100 metres. If the second camera catches you there after only 5 seconds, expect a letter

Thank goodness you don't do the sums for National Highways The average isn't 40 mph and the time taken at the limit is 5.97 s. Not much difference practically in this scenario, but significant elsewhere.

For example, if the limit in an average section is 50 mph, and you complete the first half distance at 40 mph, then you can do 70 mph in the second half for an average of 50.9 mph.

Edited by speedking31 on Wednesday 8th April 01:51

It's a very good thing for so many reasons and thanks for catching this one. I'm still a little confused though as the earlier posters seemed to be implying that the cameras were measuring your speed at each point and then averaging that, I understood that they are just timing the difference between them?

[BertBert]

zbc said:

It's a very good thing for so many reasons and thanks for catching this one. I'm still a little confused though as the earlier posters seemed to be implying that the cameras were measuring your speed at each point and then averaging that, I understood that they are just timing the difference between them?

Yes, average speed cameras use distance between pairs along with the time taken to travel that distance from which the average speed between the pairs is calculated. If the average speed is greater or equal to limit plus 10% plus 2 then enforcement can take place.