Copyright © 2002, Bernd Felsche, Western Australia

This page presents an automated method of detecting tailgating for the purpose of education or enforcement.
The method is suited to locations where there are overhead structures, such as in tunnels or under bridges.


Research indicates that tailgating is one of the main causes of speed-related crashes on congested roads; moreso than exceeding posted speed limits. Tailgating is supposedly difficult to police, with video cameras required to obtain evidence of observations by enforcement officials.

The recommended minimum space between vehicles is generally accepted to be defined by a 2-second gap; i.e. the following vehicle passes the same fixed point on the road as a leading vehicle in no less than 2 seconds.

Enforcement typically takes place when the gap is significantly less than 1 second; i.e. less than half the safe distance.

The observer has to take into account if vehicles are already braking when the measurement is taking place.

Diagram showing layout


Infra-red light beams (LASER) emitted from locations A and B above are reflected by surface reflectors aa and bb respectively. Reflected light is detected at A and B.

Control unit C processes the interruptions of the beams and signals the display unit D to display a warning message if following distances are insufficient, and/or records an image of the offending vehicle.

Operating Principle

As a leading vehicle clears the beam between B and bb, a timer (T1) starts within the control unit until the beam between A and aa is broken. A second timer (T2) start and waits for the vehicle to pass through the beam between B and bb.

If the sum of T1 and T2 is insufficient, then the vehicle currently interrupting both beams appears to be following too closely.

However; the vehicle may be braking already to increase its gap. To determine if this is the case, the control unit has to wait until the beam at A is again establised and measure the time interval (T3) until the beam at B is established. If T3 is significantly greater than T2, then the vehicle was slowing during measurement.

The control unit can then decide if it should cause a warning display to appear to the driver of that vehicle, or cause photgraphic evidence to be taken of the event.

Because there are two beams that are interrupted by a vehicle, the controller is able to estimate the maximum reasonable time for each of its timers to run; else it can assume that a vehicle, or possibly two vehicles have come to rest above the reflectors.

The system can be used to detect vehicles reversing and to alert other drivers as well as enforcement officials.


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Copyright © 2002, Bernd Felsche, Western Australia