After the start of green, vehicles arriving on a link add to the back of the queue, whilst other vehicles clear the stop-line from the front of the queue. Where blocking back may be a problem, the time when the back of the queue is most likely to interfere with an upstream junction occurs around the time when the queue is finally clearing. The last delayed vehicles will come to a stop for just a few seconds in the TRANSYT model (a partial stop), whereas real vehicles would merely slow down as they join the tail of the clearing queue, before accelerating away again. The MMQ is the estimated mean number of vehicles (or pcus) which have added onto the back of the queue up to the time when the queue finally clears.
This part of the TRANSYT model is not fully realistic. Modelled traffic is taken to travel the full length of the link at cruise speed before adding onto the queue. This may best be thought of as traffic adding onto a vertical heap at the stop-line, rather than the real situation in which the queue stretches back up the link towards the upstream junction. Consequently, the time in the green at which the maximum queue occurs is somewhat later in TRANSYT than in reality.
The other difficulty in interpreting the MMQ value is due to it being a mean value. It will therefore be exceeded in 50% of cycles during the period being modelled. The shape of the distribution of queues about the MMQ is not known, nor is its standard deviation. It is not possible to quote the value for the queue which will occur in (say) the worst 5% of cycles. One reason for this lack of a predictive formula for the distribution of queues is that it depends partly on the degree of saturation of the feeding links. If the upstream feeder links are nearly 100% saturated, then there is less scope for cycle-to-cycle variation in arrival flow downstream. The converse is true for feeder links with low degrees of saturation.
As a rule of thumb, and it is no more than that, extreme queues could well be 50-100% bigger than the mean value output; the 100% extra being for small MMQ values (single-digit MMQs).
The MMQ value is particularly designed for use with the Limit Queue facility which is set up using a card type 38. Allowance for cycles with more extreme queues than the mean will be necessary when choosing a limit queue. A value of two-thirds or half the actual storage capacity of a link is often used so that, if the MMQ can be accommodated within this limit, there is still some extra storage for more extreme queues.
For links which are more than 100% saturated, the MMQ is the mean of a queue which is steadily increasing over the modelled period. Thus, at the end of the period, the final mean queue will approximate to twice the MMQ given in the output. You will need to look carefully for such problems when interpreting your results. This is when you have to earn our reputation as traffic signal engineers: TRANSYT is a tool and not a wonder black box which always gives the right answers!