A theoretical model has been developed and used to investigate the economic justification for the installation of a with-flow bus lane along a link of a road terminating in a signalised intersection. The model takes account of the different behaviour of priority and non-priority vehicles in the various parts of the link: at the merge point at the beginning of the reserved lane, along the section containing the reserved lane, and in the queueing space at the signalised junction beyond the reserved lane. The reserved lane can be stopped short of the stop line in order to provide a 'setback' space which can be used by all traffic, thus preserving the capacity of the junction, but if this is not done and congestion results the model allows vehicles to divert along minor roads parallel to the main link if this proves to be beneficial. The benefits to priority vehicles in terms of time and operating cost savings and the disbenefits to non-priority vehicles were calculated for a range of traffic flows, compositions, signal settings and road layout; from these figures warrants (i.e. the minimum flows of priority vehicles required if the schemes are to give positive benefits) were obtained for bus lanes and for bus/taxi lanes. The results indicate that appreciable net benefits can be obtained when the degree of saturation is very high (over 90 per cent), the priority flow is high and/or is a setback of optimum length. When the degree of saturation is low, there is likely to be no appreciable benefit or disbenefit when an optimum setback is provided, but when no setback is provided substantial disbenefits can occur even at low flows. Under ideal conditions (high degree of saturation, optimum setback) only a few buses an hour are required to justify provision of a reserved lane, but under adverse conditions (i.e. where the lane gives rise to considerable congestion) the required bus flow may well exceed the maximum practicable flow for a single lane. An appendix contains a manual for assessing the merits of reserved lane treatment, which permits the results of the study to be generalised to particular situations, including schemes which give priority to classes of vehicles other than buses or taxis. (Author/publisher)
Samenvatting