1. Field of the Invention
This invention relates to a road traffic signalling system, and in particular to a system for signalling individually to vehicles in a flow of traffic.
2. Description of Related Art
A major factor influencing road safety is the difficulty drivers of vehicles face in assessing distance, in particular a safe distance to the vehicle ahead. In conditions of poor visibility, such as fog, the problem is exacerbated by the lack of visual reference points from which a driver can judge his speed and it is well established that multiple crashes are often caused when successive vehicles in a flow of traffic become too closely spaced in relation to their speed.
International (PCT) Patent Publication No. WO-88/07560 describes a vehicle guidance and proximity warning system in which a series of "cat's eye" units embedded in the road are interconnected by optical fibres. The light received by any one unit from the headlights of an approaching vehicle is transmitted to neighbouring units. By transmitting light in a forward direction (relative to the direction of travel) the path of the road ahead is lit up. Alternatively, by transmitting light in a rearward direction, a warning is provided to a following vehicle of the vehicle ahead. Although the rearward lights provide an improved indication of traffic ahead in conditions of poor visibility, a simple passive system of this type can make no assessment of vehicle speed--an essential factor in determining the safe distance between vehicles.
A more complicated system, described in United Kingdom Patent Specification No. 1,090,091, provides for the illumination of lights in "cat's eye" units both ahead of and behind a vehicle. Two forward lights are arranged to come on after respective fixed delays from the detection of a vehicle, so that if the vehicle is exceeding a speed limit determined by the time delay and the separation of the light units, the vehicle will literally "over-ride" the lights and they will not be visible to the driver. Thus, although vehicle speed is not itself measured, an indication of speed in excess of a preset limit is provided. Each vehicle detected also causes to be set up a fixed pattern of "tail" lights to the rear of the vehicle. These tail lights are colour-coded according to the distance from the vehicle, so that a following driver receives an indication of his distance from the vehicle ahead as he closes up on it. However, the distance signals are fixed and take no account of vehicle speed. Thus, in order to judge a safe distance from the vehicle ahead, and assuming he does not over-ride his own forward lights, a driver has to take account of his own speed and the distance to the vehicle ahead, based on which of the tail signals associated with that vehicle are visible to him. There is no direct indication of safe distance as determined from measurement of vehicle speed.