How do speed laser guns differentiate between two cars?

[RadPhi]

If two cars are traveling either in parallel or in close succession to one another how can a laser determine which car it is gauging the speed of?

 

[mathman]

Laser beams are very narrow. When properly used, the operator can tell the cars apart.

 

[davenn]

even more simply, because of their narrow beam, they just aim at the vehicle they want to measure the speed of

 

[phinds]

As has been said, laser guns do not have the problem you seem to think but I believe radar guns do.

 

[CWatters]

It's not clear that laser guns are fault free..

Perhaps this is relevant..

http://www.dailymail.co.uk/news/article-365563/The-great-speed-gun-scandal.html

Imported from America, the LTI 20.20. is used in nearly 3,500 mobile speed units hidden in police vans or cars and mounted on motorbikes.

In other tests, we found the equipment was measuring the speed of overtaking cars instead of the one being targeted.

Dr Clark says that the gun is defective because its wide beam can easily pick up the wrong vehicle. Furthermore, if the device is not held firmly on the target - and this is a difficult task - it can produce an erroneous speed result by "slippage".

I believe the problem of "slippage" is also known as "sliding" and it occurs when the beam slides down the sloping hood/bonnet of the car making it appear to be moving faster than it is. I believe officers are trained to focus the beam on a vertically flat surface like a number plate for this reason.