16.1

Unlike the radar devices, time/distance calculating equipment does not require type approval. However, as new equipment becomes available it will be evaluated by various forces and recommendations as to its suitability submitted to ACPO Standing Sub Committee on Road Traffic Enforcement Technology who will advise as to its future use.

16.2

A number of these devices are capable of being linked into video recording equipment. This in no way effects the operation of the device but merely provides pictorial evidence of that observed through the camera and evidence on tape of the operation of the time distance mechanism. It is imperative however that PSDB have approved the interface linkage between the device and the video.

17.1

The detection of speeding offences using the equation:

Initially this was carried out by use of a certified stopwatch and the odometer of the patrol car. Whilst this principle remains, technology has allowed for the introduction of sophisticated equipment which has proved to be far more efficient and accurate. The operation of the devices simply involves the operation of a distance switch and a time switch, in the appropriate sequence. This calls for a high level of accuracy and in order to achieve it, a good deal of practice is required.

17.2

The checking of a target vehicles speed can be done by a number of methods.

17.3

Where a vehicle is, in the opinion of a police officer, travelling at an excessive speed the device is used as corroboration.

17.4

In order that this can be done, the target vehicle needs to be checked between two reference points. The points used are best defined as "an identifiable point on or near the carriageway, either permanent or temporary:-
a) Which either casts a shadow so as to strike a moving vehicle, or which is so placed that the shadow of a moving vehicle strikes it or its shadow, or
b) where there is physical contact between the road wheels of the moving vehicle and a reference point e.g. change of surface, drain or
(c) where the target vehicle and the police vehicle are side by side

There are five alternatives:-
(i) a moving shadow on a stationary object.
(ii) a moving object on a stationary shadow.
(iii) a shadow-to-shadow contact.
(iv) physical contact of vehicle wheels with reference point.
(v) both vehicles are side by side. By using reference points which fit this definition we can eliminate any uncertainty.

17.5

It is important to remember that the police vehicle MUST pass between the same points as the target vehicle. This is essential as the distance information is taken from the police vehicle. When the police vehicle has travelled between the two points and the distance switch operated, on at the first point and off at the second point, the device then knows the distance between the points. As the target vehicle travels between the same two points, the time switch is operated. The switch being switched on at the first point and oft at the second point. The device now knows the time the target vehicle took to travel between the two points.

17.6

The device now has both parts of the equation required to calculate the target vehicle's speed. This is done in a fraction of a second and visible on a digital display.

17.7

If for any reason the operator has any doubt as to the validity of the check, it must be aborted.

18.1

FOLLOW CHECKS.

This type of check is carried out when the police vehicle is following the target vehicle.

As the target vehicle passes the first reference point, the time switch is turned on.

As the police vehicle passes the same reference point, the distance switch is turned on.

As the target vehicle passes the second reference point, the time switch is turned off. The device now knows the time the target vehicle took to travel between the two reference points.

As the police vehicle passes the second reference point, the distance switch is turned off. The device now knows the distance between the two reference points that the target vehicle has travelled.

18.2

PRE FED DISTANCE CHECK

This check is carried out when the police vehicle travels between the two reference points. Turning the distance switch on at the first point and off at the second point.

The police vehicle then parks in a position that the two reference points can be clearly seen by the operator.

As the device knows the distance between the reference points, as a target vehicle passes between the points the time switch is turned on and off. The device can then calculate the speed. Providing the distance information is not cleared from the device the police vehicle can remain parked and, by clearing the time information only, another target vehicle can be checked.

18.3

BEING FOLLOWED CHECK

This check is carried out when the police vehicle is travelling in front of the target vehicle, that is the target vehicle is approaching from behind at a speed which is excessive,

The police vehicle will travel between the two reference points, turning the distance switch on and off. The device now knows the distance travelled.

As the target the vehicle, still approaching from the rear, passes the same points the time switch is turned on and off. The device now knows the time taken to travel between the reference points. The device knows the distance and time and can calculate the speed. NB When using this method operators should guard against any suggestion of being "agent provocateur".

18.4

CROSSING CHECK

This check is carried out when the police vehicle is stationary at the mouth of a junction. The target vehicle is seen approaching crossing the front of the police vehicle.

As the target vehicle passes the front of the police vehicle the time switch is operated at an identifiable reference point. The police vehicle then moves off behind the target vehicle, operating the distance switch at the same reference point.

With the police vehicle now following the target vehicle and both time and distance switched on, the completion of the check is the same as the follow check".

18.5

DIAL IN DISTANCE

This mode enables a known distance previously measured by the device to be fed into the computer without the need to drive again over the route. NB Particular care must be exercised where shadows are used as these will change with the movement of the sun. Simply record the distance on the same push button switches as used for calibration purposes. This procedure can also be used with the control module removed from the car and plugged into the portable battery pack.

18.6

THE MINIMUM DISTANCE FOR ANY CHECK IS GENERALLY1/8 OR .125 OF A MILE.

However checks over a shorter distance, down to an absolute minimum of 0.07 of a mile are permissable under the following circumstances:-

a) The general minimum can only be reduced for Pre-fed or Dial-in Distance Checks.
b) The reference points at both the start and end of the check are physical features on the road surface which provide a clear and visual contact. (e.g.. white line/change of surface) NOT shadows.
c) The maximum speed limit at the site does not exceed 40mph.

trainers are to be satisfied that operators carrying out reduced distance checks are capable of performing the function professionally (i.e. maybe specialists for urban work)

Over short distance checks operators should consider other type approved devices that will provide a specific measurement.

19.1

In order to maintain accuracy these devices must be calibrated in accordance with the manufacturer's instructions. Full calibration MUST be carried out (a) On initial installation, (b) Following removal and reinstatement, (c) Following the fitment of new tyres and (d) IN ANY CASE AT WEEKLY INTERVALS.

19.2

In order to establish the ongoing accuracy of the device a check MUST be carried out at the commencement of each tour of duty and at the end of that tour of duty if it has been used to detect an offence. This will be either (a) Calibration check (all types), (b) Double Switching (VASCAR/SPDM).

19.3

If the device is found to be inaccurate, then not only must the defect be remedied but any offences detected since the previous accuracy check should be reviewed as it is not known at what point the equipment became defective.