In the UK, speed cameras are usually accompanied by white striped lines on the road. I have always assumed that was a visual reminder to the motorist to "watch your speed- it's about to be checked".

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However speed cameras can only detect speeds of traffic moving away from them. So on a two-lane road with traffic going both ways, it will only ever detect speeds from one of the lanes. So the white stripes seem superfluous.

What is the psychology behind putting these lines on the road, even when the camera is not pointing at them?

  • From a driving point of view it does also serve the purpose of warning you that there are speed cameras (ie on motorway gantries - where they are not obvious) and they are where you should be looking (the road) rather than where the actual warning signs are (the edges of the gantries)
    – PhillipW
    Sep 25, 2013 at 19:30
  • I wondered whether it was simply to put a little bit of doubt in the motorist's mind- "Am I sure it won't go off when I speed through it in this direction. Maybe better not risk it."
    – Urbycoz
    Sep 26, 2013 at 9:09
  • "The white stripes 'seem' superfluous" There is your error. This is not a UX question.
    – JamesRyan
    Sep 26, 2013 at 10:09
  • 1
    @JamesRyan A question involving modifiying the design of petrol forecourts to make them more efficient for the user, and thus more usable, seems like a perfectly on topic question to me.
    – Racheet
    Sep 27, 2013 at 12:51
  • 1
    To me a petrol forecourt is an interface, meanwhile a chemical reaction clearly isn't. There's no human, and proper design isn't going to improve anythings usability.
    – Racheet
    Sep 27, 2013 at 13:01

3 Answers 3


Early speed camera designs made use of radar beams only. These have been shown to provide reading with high errors, sometimes ridiculous ones. In some countries, like Israel, this has resulted in a legal turmoil, where at some point (with the help of specialised legal firms) not a single offender was convicted in court. This has prompt a revision to the law and the calibration procedure of these cameras. But the problem was not quite solved.

To combat the issue, manufacturers have added a 2-frame camera (usually with 0.5s time interval) to the already existing radar system. The white lines on the road are used as mere rulers. Since the car will hide the ruler on the lane it is driving on, there's also a ruler on the adjacent lane. It just happens that if there's only one lane per direction, the adjacent lane will be on the opposite direction (but same direction if there're two lanes in each direction).

A photo of the speed camera on kingsland road

It seems that in the UK and Australia, a conviction is made if the error between the photo analysis and the radar reading is within a 10% margin.

You can read much more about this here.

  • 1
    They also do this with large think lines, so that planes can see from the sky, that if you go from line A to line B within X amount of time, then you're going Y speed. Lots of places around here have signs that the speed limit is enforced by air patrol.
    – Mike
    Sep 25, 2013 at 17:31

Are you sure it's purely psychological and not technical ?
With 2 photos taken at determined interval you could prove/verify whenever you want the speed of the vehicles.

Maybe it's only psychological, maybe it's technical, maybe it's forward thinking. There could be an upgrade someday.

In France we have front and back speed camera, single and multiple lane. The last generation can even detect car vs large goods vehicle. So you could not trust speed camera to only detect some car in specific condition in the future.

  • No, I'm not sure whether it's psychological. I just couldn't think of a technical reason- they seem unnecessary.
    – Urbycoz
    Sep 26, 2013 at 7:59
  • Seems unlikely to be "forward thinking". How hard is it to draw lines on the road?
    – Urbycoz
    Sep 26, 2013 at 7:59

The lines are used with Gatso speed cameras as a secondary check of distance travelled. They are precisely laid in the road surface and the Gatso takes 2 pictures 0.5 sec apart. Whilst the Gatso will register the speed from the radar (and only trigger the camera if the pre-set limit is exceeded) the number of lines uncovered between the two pictures will show the distance covered in 0.5 sec and will be used as a confirmation of the radar detected speed. The reason for lines on the other side of the road is for use when a vehicle is overtaking past a speed camera. Having a set of lines in the opposing lane removes the possibility of parallex errror

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