From the above it can be seen that the value of saturation flow dictates how many vehicles can get across a given stopline within the green time (hence capacity). Stated the other way, the saturation flow dictates the length of green time needed to cater for a specific vehicular flow-rate.
TRL is only too aware of the controversial nature of saturation flows: in Research Report RR274 – ‘The use of TRANSYT at signalised roundabouts’ there were two examples quoted of saturation flows as measured at a particular roundabout. These ‘examples’ seemed to become the de-facto standard for many. There have been many occasions when engineers have asked for confirmation that they are the values to use whilst others would want us to confirm that higher values were acceptable! Our advice now is to refer to the revised text that appears in the latest TRANSYT User Guide. The guide does not refer at all to saturation flow values, leaving it up to the traffic engineer to decide what values to use and to argue the case as necessary.
We are also often asked (as we were at the User Group meeting) whether RR67 ‘The prediction of saturation flows for road junctions controlled by traffic signals’ gives sensible answers for most situations, given the age of the data it was based upon (now more than 20 years old) on the one hand, but on the other its seemingly optimistic values that some argue are not normally seen in practice One point to remember about the data collected for RR67 is that all the sites used were classified as ‘good’ or ‘average’ as originally suggested by Webster and Cobbe in their book ‘Traffic Signals’ published in 1966. The following table is a slightly modified version of the one produced by those Authors all those years ago.
Webster and Cobbe also related the expected saturation flow of good and poor sites with average: good sites would be expected to be 20% better than average, and poor sites 15% worse. The saturation flow per ‘standard’ lane for an average site at the time (mid 1960s) was 1,800 pcu/hour. However, RR67 data was based on a combination of average and good sites. Therefore the standard lane (which is defined in RR67, but not in Webster and Cobbe) value of saturation flow of 2,080 pcu/hour is likely to apply to an approach which is better than average, but not as good as ‘good’.
|Description of site characteristics
|No noticeable interference from pedestrians, parked vehicles, right-turning traffic (either owing to their absence or because of special provision made for them)
Good visibility and adequately large turning radii
Exit of adequate width and alignment
Good quality road surface
|Some characteristics of good sites and poor sites
|Average speed low Some interference from standing vehicles, pedestrians, right-turning traffic
Minor entries and exits either/both upstream or downstream
Poor visibility and/or poor alignment or intersection
Busy shopping street with high pedestrian activity
Poor road surface
Traffic calming measures on either/both entry or exit
Where congestion or downstream queuing discourages drivers from pulling away cleanly
Therefore, when it comes to estimating the saturation flow, a ‘rule-of-thumb’ for today’s conditions might work as follows:
- For a site that is definitely poor, a value calculated from RR67 may need to be reduced by something in the range 15 to 25 percent
- For a site that is average, a value calculated from RR67 may need to be reduced by something like 5 to 10 percent
- For a site that is clearly good, the range may be from the value calculated by RR67 to a value possibly greater by 10 percent
Some sites may be better classified by approach rather than site – there are situations where the main road is ‘good’ but the side roads poor. Main road straight-ahead traffic may have a saturation flow higher than the side roads or main-road turning traffic, if it has dedicated straight-ahead lanes.
[UPDATE – Since the release of TRANSYT 14, the required RR67 parameters can be specified for each lane and stored within the data file, allowing the saturation flow to be calculated by TRANSYT. As part of this new feature, the site quality factor can also be specified, and will affect the calculated saturation flow.]
MOVA sites may also have a noticeably higher saturation flow – MOVA is believed to have quite a noticeable effect on saturation flow. It appears that, over time, drivers change their behaviour, probably subconsciously, as they try to ensure they get through the current green. Saturation flows of around 2,500 pcu/hour are believed to be achieved at ‘good’ MOVA sites (and TRL measured 2,800 pcu/hour on one lane at the old Hockley cross roads near Winchester, before the M3 link between there and Southampton was completed!)
Is the above of much help when deciding what saturation flow is appropriate for any given situation? Well, the idea is to provide food for thought and ideas that you can use to back-up your own judgement, which, ultimately you will have to use. We would fight shy of giving anything that is more prescriptive than that as it can be taken in the wrong way, sometimes wantonly, other times through lack of understanding perhaps. However, we are happy to debate the issue, and if anyone cares to share any measured data that they may have collected, it may be possible to refine the way saturation flow is estimated in the future.