The diagram shows the layout of the most common type of bus set-back, and it is straightforward to model.
TRANSYT 16 and TRANSYT 17:
Bus set-backs are commonly found in traffic networks, as they are needed wherever a nearside turning movement is required for non-bus traffic, assuming that the bus lane is adjacent to the kerbside.
Buses can be modelled on a bus lane by use of a separate traffic stream. Simply specify the traffic stream “traffic type” as bus, set the bus stopped time and (bus) average cruise speed. The flow is just the buses flowing along that bus lane. The length of the traffic stream representing the bus lane should also exclude the setback length, but that adjustment is not critical.
A bus set-back looks like a flared approach to non-bus traffic and can be modelled as such. The average utilisation of the ‘bay’ needs to be estimated. Buses will start off later as a result of having to wait for any traffic in front to discharge. This can be modelled with an increased “green start adjustment” on the bus traffic stream, which will be approximately 2 seconds per vehicle that queues in front of the bus on average. (If using a link structure, buses can be modelled on a bus lane by use of a non-shared bus link with traffic type set to “Bus”.)
TRANSYT 14 and TRANSYT 15:
The bus setback can be modelled in a similar way to that used in TRANSYT 16 and TRANSYT 17, however these earlier versions of TRANSYT are missing the ability to apply “green adjustment” directly to a traffic stream. Therefore, the signal timing adjustments have to rely on the phase-based adjustments.
The effective ‘late start’ of the bus can be modelled with an increased “relative start displacement” for the bus phase, which will be approximately 2 seconds per vehicle that queues in front of the bus on average. However, this will necessitate the creation of an extra phase for the buses, in order to be able to apply the “relative start displacement” to just the buses and not to the other traffic in the adjacent lane. Make sure that the extra phase runs during the same stages as the phase controlling the other traffic, i.e. it is in effect a ‘copy’ of it, but with the relative start displacement additionally applied.
(Updated April 2025)