Summary of Detection Methods.
What do policemen use to detect speeders on the road?
Their primary method of detection is to use a laser gun. Being one of the newest technologies and unlike radar guns, a laser beam doesn't spread out much at all, so the police can pinpoint one car and get a precise reading on it. So, why don't police just use laser? You need a VERY steady hand to use a laser gun, because the beam travels so fast there and back and the computer calculates the speed based on perhaps a quarter second burst. So, if the policeman moves his arm so much as a millimetre while shooting the gun, and the beam moves to a point on the car that is closer or further away then the initial contact point, the reading will be totally off. Think about it, if a policeman is pointing the gun at a front licence plate that is 150 feet away, a tiny movement of his arm would move the beam to the windshield of the car.
Because of these problems with laser, some courts in the USA have ruled that police cannot use laser for speed detection unless they can cross-reference their speed estimate by either a stopwatch or radar. Radar guns use 3 different bands, these are X, K, and Ka super wide. There isn't a huge difference in performance of any of these bands, they just use different frequencies. Even with all the new technologies out there, police still tend to rely on their radar guns, they are tried and true. They do have a number of drawbacks, however. The beam emitted from a radar gun spreads rapidly, which means it performs poorly in heavy traffic conditions. There are several other methods of detection the police may use. One of them is a speed trap such as Autovision, VASCAR, DS2 etc. Listed in the last pages, in which a portion of road has been measured or had devices fitted in the tarmac, and when you enter the speed trap, the police start a timer, video or similar... and they can calculate your speed based on how long it takes you to get through the speed trap. These can work very well. There's also photo radar, Gatso which may be built into traffic lights in medium to large size cities. The radar beam will calculate your speed, and snap a picture of your licence plate.. and you'll get a ticket in the mail in 14 days.