A trial of an improved signalling system took place between 1985 and 1990 on the M1 motorway between junctions 10 and 19. Signal spacing was reduced to 1km and signals were switched automatically when loops, installed in the road surface every 500m, detected the presence of stationary or slow-moving traffic. The northbound carriageway was used for the trial, the southbound carriageway serving as a control. Details of all accidents were recorded by the police patrolling this section of motorway. This report describes the collection of data and the analysis of the numbers of accidents occurring in the study period. Whilst no evidence emerged that the improved signalling had reduced the numbers of accidents during the trial, a number of difficulties were encountered which probably contributed to this outcome. The effect on accidents of the nearly twofold increase in traffic during the trial was not well understood. The southbound carriageway exhibited a different accident pattern bringing into question its value as a suitable control. The trial relied heavily on damage-only accident data, known to be less reliable than injury accident data, and there were inconsistencies within the trial. The design of the trial required a 10 percent real reduction in accidents for a reduction to have been detected. The most likely source for such a reduction was in secondary accidents which would need to have been reduced by 40 per cent. Such a change was not demonstrated but this finding does not, however, preclude the possibility of a change at a lower level. Other benefits are likely to accrue from the improved signalling, particularly in driver credibility, in saving of police time and in response time to incidents. (A)

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