Mean Maximum Queue Part 2: Signalised roundabouts - TRL Software
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TRL Software > Support & Training > Articles > Mean Maximum Queue Part 2: Signalised roundabouts

Mean Maximum Queue Part 2: Signalised roundabouts

In one case, it appears that a customer subsequently specified that a signal system on a roundabout should be designed so that the MMQs were never more than 50% of the available queueing space on circulating links.

The article was intended to explain the meaning of the MMQ, and did not attempt to define precisely how TRANSYT should be used in the design process to produce timing plans. Often, practical constraints mean that any solution may be less than ideal. It is up to the design engineer to understand the modelling and optimisation process, and to make use of the various facilities (such as Limit Queues and Link Weighting Factors) to produce the best, acceptable solutions.

Defining what are, or are not, likely to be practical working solutions requires experience and judgement. The design engineer must examine the results that are output from TRANSYT to see when precisely the build-up of queues on critical links takes place, in relation to the red and green times.

Sometimes, the designer of signal plans for roundabouts/gyratories will find that MMQs which exceed the normal limit queues can be tolerated provided that the cycles when bigger-than-average queues occur do so when any blocking-back merely inhibits for a short time the passage of more traffic entering the system. Conversely, if they occur when blocking-back would inhibit the passage of circulating traffic, then this is usually unacceptable because it is likely to cause lock-up of the roundabout. What one is particularly looking out for is the well known shock-wave effect where an arriving platoon experiences a brief delay while a standing queue clears downstream. Such shock-waves cause large MMQs in the TRANSYT results, which may or may not be tolerable depending on their timing.

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