Question: Bottlenecks sometimes can appear to have excessively high arrival flow rates on the downstream links. How do i fix this?
Answer: In fact the problem is not related to the bottleneck mechanism at all, but is perhaps most likely to be recognised there. Bottlenecks are used to constrain traffic to lower saturation flows than would otherwise occur – e.g. platoons passing over a narrow bridge.
A brief statement of how TRANSYT matches up differing values of total link inflow and outflow is helpful. There is no requirement in TRANSYT for the sum of inflows and the total link flow to be the same; after all they are different junctions and will almost certainly have been measured on different days.
TRANSYT handles this situation by assuming that the link total flow is accurate, and factoring the inflows such that the sum of the factored inflows is equal to the specified total link flow.
This system is fine if the inflows and outflows are roughly equal, which is of course normally the case. If however there is a gross mis-match in the inflows, then the factoring process will of course not only raise the total flow value, but also proportionately raise the flow in each individual step. The modelling is such that the increase (or decrease) has to occur by factoring the flow in each step, and not by spreading the increased (decreased) flow across more (less) time steps. The figure shows how, in a very simple case, a link outflow profile can be higher than the previous link saturation flow due to the effects of factoring.
Thus, despite the effects of dispersion, it is possible to get traffic arriving at a downstream stop line at a higher rate than the saturation flow at the upstream stop line. This is clearly impossible, and warns that the model needs to be checked. First check that all inflows have been specified – if one is missing, this can cause substantial factoring of the others! Otherwise it is just a matter of reconciling the flow differences.