An assessment of the effectiveness of the speed discrimination feature of traffic signal systems installed at seven dual carriageway junctions has been attempted using behavioural measures. Speed discrimination is designed to extend the green phase by appropriate amounts to allow drivers, who might otherwise be caught in the 'dilemma zone' at the onset of the amber signal, sufficient time to clear the signals before the red appears. During a short period when the speed discrimination was disconnected no significant increases in the occurrence of traffic conflicts were observed; statistically significant increases in the frequency of incidents involving drivers who ran through a red signal or who braked heavily were, however, detected at two sites. The speed and approximate position of drivers at the onset of amber who subsequently continued through a red signal were analysed and the results are discussed. At several sites where speed discrimination had more opportunity to play its role, the systems were apparently failing to account for all drivers in the dilemma zone. There was also a large proportion of drivers caught in the dilemma zone due to maximum green changes, and appreciable numbers deliberately running the red even though they could have stopped comfortably. (A)

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