TRANSYT 14 and TRANSYT 15
If using the platoon dispersion model (PDM or Quick PDM) In TRANSYT you simply need to set the “Exclude from PI calculation” flag in the “Modelling” section of the “Link data or “Traffic Stream” data. This will ensure that optimised timings will excluded the PI for this link from the performance index calculation, and timings will therefore be the same as before you added the exit links. Instead of excluding the results from the P.I. alternatively you can define the exit link or traffic stream as having no restricted flow (e.g. by adding an unrestricted link or traffic stream in NetCon). This ensures that traffic remains unimpeded no matter what level of flow travels along it.
If the exit link or traffic stream is modelled using the cell transmission model (CTM) you need to be aware that CTM restricts traffic naturally at both ends of the link (e.g. at the stopline, give-way line AND also at the upstream end of the link or traffic stream. This upstream restriction (cell saturation flow) determines the maximum amount of traffic that can enter and travel along the link (as opposed to the saturation flow – that defines any restriction at the downstream end). Therefore it is recommended that a suitable cell saturation flow is selected to reflect the number of lanes it represents. It follows that even if the link or traffic stream is defined as being excluded from the P.I. calculation AND/OR has no restricted flow (at the downstream end) it can still act as a restriction to traffic entering and can result in blocking back . This, of course, is correct (modelling-wise), but if you do not want this restriction to occur you should set an overly high cell saturation flow to remove this effect.
Please note that, in order to avoid mistakes regarding the setting of Cell Saturation Flow values, a warning message is reported if a traffic stream that represents two or more lanes also has a cell saturation flow left set to the TRANSYT default of 1800 PCU per hour.
In TRANSYT 13 you simply need to set the “Exclude from PI calculation” flag in the “Modelling” section of the “Link Data”. This will ensure that optimised timings will excluded the PI for this link from the performance index calculation, and timings will therefore be the same as before you added the exit links.
Please note however that if using the Cell Transmission Model (instead of the platoon dispersion model), traffic can be restrained from entering the link at its upstream end – this is part of the nature of CTM and why it can model blocking back.
In TRANSYT 12 adding a new link WILL affect the results, but as long as you specify the link as a bottleneck and specify zero stop and delay weightings (i.e. -9999 values) you will obtain the same signal timings.
N.B. Some values related to the total length of the network, such as “distance travelled” through the network will be very-slightly different. Setting the link to be as short a link as possible will reduce this effect to a minimal difference.