We've all spotted the mobile speed cameras popping up in Sheffield, but not many people know when they will be caught.
Most of the mobile speed cameras are run by local police forces and can be found on accident blackspots with a history of road traffic accidents.
The mobile speed cameras can take on a number of forms including marked or unmarked cars.
They can also be operated by police officers using radar guns and laser guns - but when are you out of range from these?
Many motorists think they can be out of range from a speed camera van and they can just slow down in time if they're over the limit.
However, this isn't strictly true.
The speed cameras and guns work by using laser technology and can see for around two miles on a straight section.
This is far longer than the line of sight for most locations that safety camera partnerships will patrol.
While the cameras can't record around bends or over hills, as long as the camera can target your vehicle they can get a reading of your speed.
The RAC said this is because the laser beam emitted by a mobile speed camera when it hits your vehicle can be as small as 10cm in diameter.
Drivers also can't hide behind another vehicle to avoid being seen or avoid detection while driving on the opposite side of the road.
An RAC spokesperson said: "Speed cameras are one of the most contentious points of motoring for a great many drivers.
"Whether you see their benefits or not, you'll have to learn to live with them, so it is advisable to know all you can on the topic to ensure you stay on the safe side of the law.
"If the speed limit changes you have to be travelling at that speed the moment you are within the limit boundaries (as soon as you pass the sign).
"This means if a speed limit changes from 40 to 30 you will have to bring your speed down to 30 in advance of entering the zone, not begin slowing down as you enter it."