Trrl Lr 814

Normal traffic signal operation can be modified to give buses priority, if buses are detected separately from other vehicles. A computer simulation has been used to estimate benefits to buses and disbenefits to other traffic when using various forms of priority control superimposed upon the normal UK vehicle-actuated signal operation. All the cases simulated were for two-stage signals controlling crossroads-type intersections with buses on the major road only. The facilities considered are: a 'priority call' which regains the green for buses approaching a red signal, and a 'priority extension' which maintains an existing green. The priority call curtails the green time on the non-priority stage, and various methods are considered for alleviating disbenefits to this traffic. Simulation results form the basis for guidence in the use of different methods of priority control with various traffic conditions, bus detector locations and bus flows. Appropriate priority control can usually achieve delay to other travellers. It is less easy to attain a net benefit if equipment costs and the high time value of non-priority vehicle occupants such as commercial vehicle drivers are taken into account.