Question about how speed cameras work

ubersonic · 17 Sep 2022 at 14:17

Hi all,

I saw something I didn't understand on the M6 yesterday, the smart gantry system had changed to 40 MPH so I slowed down to this on approach resulting in a speeder in the middle lane (~50 MPH) overtaking me while at the same time another speeder in the outer lane (~60 MPH) overtook him. They both passed under the gantry setting off the system resulting in a double flash, but the way I expected it to work is there should have been four flashes? (As the faster speeder should have tripped the system first then the slower speeder tripped it again).

So yeah, can one gantry target all four lanes at once with a single double photo? That seems odd to me as it didn't look like they were both in the floor markings at the same time, but if so how does that work? Or does the system have a reset time and the second guy got lucky? Or are they simply set not to trigger unless the target is more than 10 MPH over?

lsg1r · 17 Sep 2022 at 14:49

Technically on a fixed gantry system you don't even need the flashing lights to determine the speeds of a vehicle and it could just be a case of 'caught you' lighting. You can work out speed by markings on the road or on the 'camera lens' showing distance over a set time (there's likely 'radar' too).

Think of it a bit like how they decide who won in a photo finish.

There could also be some tolerance in the trigger margins, no speedo is 100% accurate, like you mention... it depends on how much money the local council wants lol

Twinz · 17 Sep 2022 at 14:51

Not sure how they do it over there, but in the states there are sensors in the pavement in each lane that trigger the picture. It could be as simple as an overlay on the photo that indicates which lane produced the speed reading. Even there is a car in each lane, the speed *and* the lane can be matched up.

GeX · 17 Sep 2022 at 18:37

Usually 10% + 2.5mph before they go off, so 46.5mph if showing 40. Car speedometer will over-read
“speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph.” and so that car doing around 50mph likely didn’t trigger it, only the 60mph one did

theone8181 · 17 Sep 2022 at 20:20

I thought there was something like a minutes grace when the limit changed on those things.

BUDFORCE · 18 Sep 2022 at 09:38

GeX said:
Usually 10% + 2.5mph before they go off, so 46.5mph if showing 40. Car speedometer will over-read
“speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph.” and so that car doing around 50mph likely didn’t trigger it, only the 60mph one did

This would me my guess as well.

Nemiyen · 18 Sep 2022 at 09:46

theone8181 said:
I thought there was something like a minutes grace when the limit changed on those things.

You would hope so, I've seen people driving along the motorway and then it's activated a reduced speed right as they're under it and people all slam their breaks on while they were doing 80mph etc. I know they should be doing 70mph but even from 70mph it can cause a major accident if everyone just slams their breaks on at once, especially tailgating lorries overtaking.

They need to start flashing or have a dot timeout around them so it gives people time to slow to that speed, not instantly drop to it from 70/80mph!