Question: I have a small network of signalised junctions that are quite a distance from each other. Is there a particular distance (or rule of thumb) beyond which coordinating the signals using TRANSYT is of no benefit?
Answer: No – there is no particular distance. The usefulness of TRANSYT will depend not on the distance between junctions but on how much the platooning of traffic remains intact along the link by the time it reaches the downstream stop line. Platooning is affected not just by distance but also by the amount of skin-friction along the link due to characteristics such as the presence of parked cars, bends or minor sources and sinks. The presence of more significant priority junctions can also contribute to this if they add significantly to the traffic already on the link. Although in this case you would probably want to model the give-way within TRANSYT too. Platooning is also significantly affected by the nature of the upstream junction. Good platooning is likely only if there is a dominant movement at that junction.
The only sure way to know how intact platoons remain along a link is to go out on-street and see what is actually happening by measuring cyclic flow profiles on street. The degree of platooning is easily seen from the data collected. Within TRANSYT itself the degree of platooning is indicated by the value of MME (Mean Modulus of Error). The MME value is given in the cyclic flow profile graphs. Whilst platoon dispersion due to differing speeds is modelled by TRANSYT, skin friction effects are not. So MME is a theoretical value. A value of MME close to its maximum of 2 indicates a high level of platooning so good coordination will be effective while a value of 0 indicates no platooning at all and hence no coordination benefits are possible. A ‘rule-of-thumb’ is that for any link with a MME value of 0.3 or less, it is probably not worth coordinating the signals. However, if on links, whether short or long, skin friction effects are significant, the MME value is not likely to be meaningful.
N.B. This article revised – Nov 2008