VASCAR (Visual Average Speed Computer And Recorder) is nothing more than a glorified stopwatch coupled to a computer and the police car odometer. The officer manually starts/stops the 'stopwatch' when a vehicle passes between two reference points that are a known distance apart. The speed of the vehicle is then calculated by dividing the distance by the time.
Distance over time devices are exactly that. They measure the distance an object travels while timing how long that trip took. They then divide the distance by the time to calculate the speed of the object...literally "distance OVER time".
VASCAR can gather the time and distance measurements in two distinct ways:
1. Pre-fed distance checks
The officer themself drives between two reference points (A and B) measuring the distance as they go. The officer then parks up and times cars traveling between points A and B. The computer will then display the speed each time the timer is stopped (at Point B).
2. Follow checks
The officer is traveling in their car behind (or in front - see below) you. The officer has NOT had opportunity to measure the distance between two reference points (unlike in the above "pre-fed" example) as they themselves are also moving.
As you pass a suitable reference point on the road (point A) the officer begins the time measurement. When the officer passes point A, they begin the distance measurement. Once you pass another suitable reference point (point B) they stop the timer. Once they pass point B they stop the distance measurement.
As the device now has both the time and the distance measurements your speed can be calculated.
This can also work in reverse where the officer is in front of you and watches you in their rearview mirror, the only difference is the order of commencing the distance and time measurement are reversed: i.e. the distance measurement is started first and then the time measurement.
When operating the VASCAR device a "Reference Point" is used. This is defined by the ACPO as an "identifiable points on or near the carriageway either permanent or temporary." Examples include white lines on the road, a bridge, a shadow, a join in the tarmac, a road sign, pretty much anything!
No Home Office Approval: Distance over time devices do not require home office approval because they work by an officer visually timing a car between two reference points that are a known distance apart. High school physics of Speed = (distance / time) gives us the vehicles speed. There is literally nothing about the device to test or approve.
Incidently, LIDAR based devices can be considered distance over time devices, but these devices deduce the distance indirectly and therefore require home office approval.
Completely Undetectable: As distance over time devices do not use any electromagnetic waves they are completely invisible to all RADAR/LIDAR detectors.
Cheap: The device is cheap to install and cheap to service.
Whenever You See A Police Car: you're best off assuming it has a VASCAR device installed.
Officers can appear to be traveling down the road inconspicuously when in fact they are constantly performing speed checks. Similarly an officer who is genuinely minding his own business can quickly perform a speed check should they notice a speeding car.
Distance over Time devices must be calibrated when they're first installed in a vehicle. They must be checked (and re-calibrated if necessary) once a week as well as at the start and end of each officers shift.
Interestingly, if the tyres of the vehicle is are changed a check/re-calibration must be performed.
Using the VASCAR device has a significant margin of error due to the human judgment of when the target and officer vehicle's pass reference points A and B. To mitigate this, the minimum distance between point A and B must be 0.125 miles (one-eighth of a mile or 200 meters). The longer this distance the less impact any error made by the officer will have on the speed calculation.
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