VASCAR units deactivate automatically after 7 days which means the operators have recalibrate them, this however is not the case and most officers test the unit at the start of each shift. Common places for the use of VASCAR is on motorways, in which the police car will wait just out of sight on a slip road, and time the vehicle between two points, usually the white painted square or round markings on the road, but any fixed points can be used, that the distance between them is known. They can also use it when they are following you or when you are following them.

The minimum distance the police are allowed to is 0.125 miles - one-eighth of a mile. Under exceptional circumstances, they're allowed to go down to 0.07 miles but only in pre-fed or dial-in distance modes where the start and end points are not shadows. Police helicopters and Motorbikes can take the same measurements

Those white squares and circles that are painted on the road are used for VASCAR systems. They are set distances apart, and quite often are quite visible to the police from a great distance, they could be parked up to half a mile away and still get a speed reading of an offending vehicle. But remember any fixed point can be used, such as a lamppost.

This is potentially the easiest one to throw in court, but can be a hard job to do. It is open to human error on the timing positions and the pressing of the button.

Just a little note sent to me from a Traffic Officer."If Vascar is used correctly the margin of error is presumed to be the same at either end of the check.
The Home Office allows a percentage of error for the overall check and this built into the operating procedure. Each and every police officer undergoes an intensive training course to establish their margin of error. If it falls within the guide lines then it is "acceptable". So don't go using the info below unless you can prove your case without doubt."

Extract from USA Law Paper

VASCAR (Visual Average Speed Computer and Recorder) is really a stopwatch coupled with a calculator. Once a distance is entered, the operator pushes a button to start then stop the stopwatch. VASCAR displays the speed calculated from the distance entered and the time measured.

Thus, a VASCAR measurement depends on human input. Therefore, in order to make a VASCAR measurement as accurate as possible, the observer's/operator's reaction time must be as short as possible. (Reaction time is defined as "the time interval between an input signal (physiological) or a stimulus (psycho physiological) and the response elicited by the signal.)

What is considered "normal" human reaction time? First, let's take a look at the best: Drag racers. Average reaction time of (Top Fuel) drag racers at the 1997 U.S. Nationals was 0.124 ± 0.082 second. Imagine that our cop running VASCAR is a super cop, with drag racer like reaction times: What kind of error is introduced into the VASCAR speed measurement by a reaction time of 0.124 second? With the numbers testified to by our cop:

Distance ÷ Real time ± Error = Measured Speed 100 ÷ 1.54 + 0.124= 60.096 feet/second (40.97 mph)
or
100 ÷ 1.54 - 0.124= 70.621 feet/second (48.15 mph)
Depending on whether the cop was 0.124 second too quick or too late.

The calculation above takes into consideration only one opportunity for error.

However, the cop must not only start the VASCAR's stopwatch, but he also must stop it. Thus, he can introduce the error twice:
100 ÷ 1.54 + 0.248= 55.928 feet/second (38.13 mph)
or
100 ÷ 1.54 - 0.248= 77.399 feet/second (52.77 mph)
Looks like we are getting some error margin here: it is almost 15 mph. But wait a minute: the cop alleged that he clocked our man 66 mph (96.8 feet/second).

If true, the elapsed time over 100 feet distance was 1.033 second. The error in speed reading introduced by human reaction time becomes even more severe:
100 ÷ 1.033 + 0.248= 78.064 feet/second (53.225 mph)
or
100 ÷ 1.033 - 0.248= 127.388 feet/second (86.855 mph)

WOW! An error margin of more than 33 mph! And remember we assumed that the super cop in this example has reaction times similar to a drag racer!

Imagine if the cop is only human: Average adults show reaction times around 0.3 second -- it's not even worthwhile to do the calculation. Obviously, we can no longer talk about accuracy and VASCAR in the same sentence. One can conclude that using VASCAR over a 100-foot distance cannot yield any accurate speed reading for the speeds alleged in our man's case. We come to this conclusion even before considering other sources of error, such as visual distortion introduced by the parallax effect.

A little note though UK Police don't take a reading over this distance, theirs is greater, so the reading could be more accurate.