The use of active speed warning signs

The problem of drivers failing to obey speed limits, even when non-compliance is dangerous, continues to present a major challenge to highway engineers. In Germany, as in the UK, attempts are being made to change drivers’ behaviour by both enforcement and persuasion. One method of persuasion being tried is the use of 'secret’ signs to warn of excessive speed. These signs are normally blank but display a warning when triggered by a speeding vehicle. The historic use of secret signs and trials of systems is presented along with a review of their use in other countries. The systems reviewed range from those which check vehicle speed to ones which check not only the speed of the vehicle but also its weight and dimensions, so that warnings specific to that vehicle can be given. A UK trial using number-plate recognition is also reviewed. Before-and-after studies have shown that in the right location these signs achieve up to 87·6% improvement in compliance with speed limits. Not all signs proved to be successful, however. The signs have been found to be most successful during their early life. As drivers become used to the signs, the effectiveness reduces. A system used in the Netherlands uses a variety of messages to overcome this problem. Vandalism has also been reported as a problem with these signs, with one in the US being destroyed by high-velocity bullets. Studies in the UK have shown that these signs can be more effective in reducing vehicle speeds than direct police enforcement using speed ‘guns’.