Here we have a situation where one link feeds two downstream links that share a common movement.
This is illustrated in the link diagram below,where link 15 feeds links 24 and 25. Assuming that there is good reason why links 24 and 25 have been separated, there is the potential for these links to be used differently depending on whether traffic from link 15 receives green or red. It may be that, during green, only one of the links is used; conversely the links may both be usable (though not necessarily equally so) during red.
Generally speaking, the assignment of traffic during green is of little consequence compared to correct assignment during red, where queueing behaviour, queue-lengths and the discharge rate of the queue are the main concerns. During green, the ‘OUT’ profile of the traffic on links 24 and 25 is governed by the ‘OUT’ profile on link 15 and not any characteristic of links 24 and 25. It does not matter whether traffic uses the nearside or offside lane, or even straddles the lane marking, the model retains its integrity.
However there may be something to look at when both upstream entries (links 10 and 15) are considered. It may be that the majority of traffic on link 10 is turning right at the next stop line; hence needs to use just link 25. Traffic from link 15 may be split in its required destination such that links 24 and 25 are equally useable on average. If the traffic from link 10 hits red on link 25, it might form a significant queue on that one link. When traffic from link 15 reaches that approach, drivers will naturally choose the lane with the smallest queue until both queues are about equal in length, thus tending to use link 24 more than link 25. On the other hand, if it is traffic from link 15 that hits the red first, it will queue evenly (or less unevenly) on both links. Hence the assignment could be different depending on the coordination.
If this presents a problem, a possible solution would be to assume that both links 24 and 25 are used by traffic from link 15 (equally or unequally as appropriate) and see whether the timings from TRANSYT mean that this traffic hits red before traffic from link 10 or not. If the traffic from link 10 hits red first, you may need to increase the proportion of link 15 traffic entering link 24. If after running TRANSYT once more, the coordination changes so that link 15 hits red first, you need to choose which of the two timings you prefer, assign traffic as required, and fix the offsets by grouping the two nodes. This way the assignment will remain correct for optimised timings produced in further runs. Notice in the solution that we are still concentrating on what happens during red and not green. Which of the links is used during green is, as before, of little consequence.
Update (2015): Although this FAQ is illustrated using a LINK structure, the issues it raises are still valid for a Traffic Stream one, and relevant to all versions of TRANSYT. The difference is that it simply easier to adjust how much traffic is using which PATH through the network when using Traffic Streams and their associated OD Matrix.