The diagram shows the layout of the most common type of bus set-back, and it is straightforward to model.
TRANSYT 14 and TRANSYT 15:
The buses can be modelled on a bus lane using a separate traffic stream. Simply specify the traffic stream “traffic type” as “Bus” and set the bus stopped time and (bus) average cruise speed. The flow is just the buses flowing along that bus lane.
A 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 “relative start displacement” for the bus phase, which will be approximately 2 seconds per vehicle that queues in front of the bus on average. N.B. This will necessitate the creating 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.
If you are using a link structure to build your network model you can more-or-less follow the advice given for TRANSYT 12 and TRANSYT 13 below.
TRANSYT 12 and TRANSYT 13:
The first important point to note is that two separate links should be used. It is not a case where shared links can be used as the two lanes have very different uses. You can still assign the bus lane as a bus link by entering a four-digit value in the cruise-speed/time field (of the format XXYY where XX is the cruise speed in km/hr and YY is the time spent at bus stop(s), including `00′)
Next, the link for the other traffic takes the form of a flared approach, and can be modelled as such in TRANSYT. Decide how many vehicles, on average, will use the space between the end of the bus lane and the stopline; then estimate the saturation flow of the short lane or bay (TRANSYT is not especially sensitive to this value). Enter these into the TRANSYT model.
Finally, the start lag for the buses needs to be adjusted to take account of the time it will take (again on average) for the first bus to cross the stopline after the start of green. This will be the normal start-lag, plus about two-seconds for each vehicle between the bus and the stopline.