Urban road networks are divided into the main road network with a general speed limit of 50 km/h and the 30 km/h-zones. The reduction of the speed limit to 30 km/h even on the main road network is under increasing discussion. The same positive experiences and benefits - already gained in 30 km/h-zones - are expected to be reached in the urban road network, too. Especially progressive signal systems on urban main roads would be affected by this measure. The study aimed to define the pros and the cons of progressive signal systems "green waves" with progression speed of 30 km/h on urban traffic flow. The research was carried out empirically by a before-and-after-study. The before-period regards a speed limit and a progression speed of 50 km/h. Two cases were examined in the after-period. On the one side low progression was enforced by a speed limit of 30 km/h on the other side a speed limit of 50 km/h was established. The chosen criteria permit an examination in respect of traffic safety, environmental impacts, capacity and traffic quality. Criteria based on vehicle-dynamics were introduced because of difficulties in the evaluation of traffic safety in short study periods. Traffic safety increased for low progression with speed limit whereas a tendency to the contrary was established in the second case. Compared to the before-period expenditure of energy and noise immission decreased for low progression with a speed limit but unchanged or risen for low progression without speed limit. Low progression without speed limit caused in homogeneous traffic flow. The differences of capacity are negligible. (A)
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