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