M27 average speed cameras: Here's how they work and how to avoid a fine

The bright yellow camera gantries, installed over the last few weeks near Portsmouth, will be used to enforce a 50mph speed limit as work starts on the M27 smart motorway scheme.

Narrow lanes will be used while work is being carried, meaning the speed limit will have to be lowered from 70mph.

Average speed cameras work by tracking the speed of your car between two points. So slowing down to go through the camera and then speeding up will not work.

Instead, the cameras take your number plate when you pass through the first camera, then your number plate again at the second, and perform a quick calculation based on the current time to work out how long it took you to travel between the two points. If the time it took you to travel is quicker than could be done at the speed limit, you'll get a fine in the post.

So if the speed limit is 30mph and you drove at 35mph for 30 seconds, you'd need to drive at 25mph for 30 seconds to balance it out.

Here are some common average speed camera myths busted...

Do the cameras work at night?

Yes. They are fitted with infrared night vision to ensure they work night and day, and in any weather. Sorry.

Can the cameras run out of film?

No. Unlike some other speed cameras, average speed cameras are all digital. So they can issue an unlimited number of fines.

If a vehicle changes lanes will they avoid any fines?

No. The cameras take pictures of every lane and cross-compare them. So changing lanes won't save you, contrary to popular belief.

If I speed through multiple sets in the same of road stretch, will I be fined multiple times?

It's not likely. Not every camera is paired to every other. So if there are four in a row, your speed between cameras 1 and 3 and 2 and 4 may be compared, or just 1 and 4, etc. But you won't know which is connected to which.

But on a separate section of the same road, you could get fined again.

If you are less than 10 per cent above the limit, will you get a ticket?

National Police Chiefs Council advice previously suggested 10 per cent as a buffer, but this is only advisory and in reality many forces are far stricter, especially as cameras become more accurate. In fact, according to the law you can legally be fined for being just 1mph over.

The best method to avoid a speeding ticket? Don't speed, of course...