Tailgating

Problem

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.

Equipment

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

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.

Limitations

• Vehicles must remain in lanes for measurement. This can be assured by continuous lane lines, as well as a detector at the measurement site to identify vehicles crossing the lines.
• Vehicles must break both beams for a valid measurement. Narrow-tracked vehicles may be able to avoid reflectors in the lane if the width of the lane is not entirely covered.
• Long vehicles may be past the display/camera location for an effective display or recording of the event. The effective display/camera angle is shown shaded in the diagram above. This can be compensated for by a second display/camera unit further down-stream that's selectively activated when long vehicles have been detected.
• False returns of detection beams can be identified by using pulses to instead of steady beams for detection. Premature reflection of a series of pulses indicates that the vehicle is reflecting/emitting light.
• An additional camera may be employed for enforcement purposes, angled so that the followed vehicle is also shown in photos.
• Reflectors are the most vulnerable components of the system. Vandalism that may occur if the system is deployed for enforcement. This can be discouraged if the system records events such as reflector outage photographically and alerts authorities.