Some pointers for congestion query
Dear Sir, — In last week’s Chronicle, Richard Hamilton (letters) wondered why the Sandbach transport study did not show any congestion near M6 junction 17. I can’t give a complete answer, but can offer some pointers.
The lack of congestion near the M6 reported by the study was the first thing we noticed that was odd and didn’t accord with reality for any Sandbach commuters.
But it was impossible to say why this was so without further information.
Despite pressure from Sandbach Town Council to provide this, some limited data was only released by Cheshire East Council in July 2017, once the local plan examination was complete.
It then took a fair amount of time to investigate the data to try and understand what had been done.
It’s mainly about artificially suppressing traffic demand in the study, which is one of the inputs to the VISSIM model developed.
Traffic demand reflects the number of vehicles wanting to go through a particular bit of road network to get from A to B.
Congestion builds up when the demand is greater than the capacity of that bit of road to cope with the demand.
Calculating demand starts with traffic surveys of what traffic is actually using the road.
We know via a freedom of information request that the Sandbach study was to rely on pre-existing traffic surveys, not carry out any specifically for the study (presumably to save time and money).
Cheshire East could have used the traffic demand developed by Highways England as part of its design of the pinch point scheme for junction 17 of the M6 (the mini-roundabout for the northbound slip-roads and the traffic lights for the southbound slip-roads).
It did not.
Instead it based this part of their model on a traffic survey carried out by the applicant developing Abbeyfields. The survey was carried out in February 2011.
One major flaw in the reporting of this survey (common to many developer surveys) is they did not report any queue lengths, resulting from local traffic congestion.
As far as we can tell, Cheshire East used this survey as it was reported, without making any allowance for any queuing traffic.
Secondly, the base model was developed for a 2015 base year, although the survey was carried out in 2011. Normally a factor is applied to account for background traffic growth over the intervening four years (the so-called Tempro factor). It is clear from the data supplied to Sandbach Town Council that no such factor was applied in this case.
Thirdly, we have the traffic growth discrepancies (whether 7% or 21%) to account for all the other developments around Sandbach. This has already been covered at length.
Fourthly, there are the traffic projections from the Capricorn business park included in the local plan as CS24. This comprises phase one, which is currently being built, and a phase 2A, for which an outline application (17/4838C) has been with Cheshire East for some time awaiting determination.
The traffic expected from phase one was included in the study (under report Option B), but no allowance was included for any traffic arising from Phase 2a. Why not? — after all it’s all part of CS24.
So there is a consistent pattern of under-representing traffic demand in the study:
• Already existing in 2011;
• From background traffic growth between 2011 and 2015;
• Expected and actual traffic growth from other developments around Sandbach; and
• Expected traffic from CS24 (Capricorn) itself.
This is all very handy if you’re trying to convince the local plan examiner that CS24 can form part of the local plan without unduly affecting the local road network.
The study also addressed capacity of the road network near M6 junction 17, particularly in terms of facilitating traffic movement along A534/Old Mill Road.
Much has been discussed about the various roundabout designs to replace the pinch-point mini-roundabout on the junction with the northbound slip-roads.
Much less has been said about the traffic light setup on the southbound slip-roads, although these have a major influence on how traffic can move.
The setup of these lights is controlled by Highways England, not Cheshire East. Highways England is responsible for the strategic road network including the M6 and implemented the pinch-point scheme.
Cheshire East changed the traffic light parameters used in the transport study from those set up by Highways England. Cheshire East has conceded as much in correspondence. It is also clear that this change was not agreed with Highways England — it was not even informed.
Significant questions remain that Cheshire East have so far failed to answer. In particular, it appears that the amount of traffic that its study shows travelling along the Old Mill Road at peak times is greater than the capacity of the traffic lights to handle.Mind you, you won’t find any discussion about the set-up of these traffic lights in the Sandbach traffic study report. In fact, they are not mentioned in the report at all. Perhaps they did not get included in the VISSIM model either. — Yours faithfully,