Bypass traffic jam.

Comments on Bypass traffic forecast

This is an expert review of the A6-M60 Business Case’s Forecasting Report. The report begins on page 219 of Appendices 4-9. There is also reference to Volume 1 of the Business Case.

  1. Forecasting Report

In line with normal practice, the Forecasting Report documents changes to the transport network and travel patterns between the present day and a future year “Do-Minimum” (or “Without Project”) scenario. It then assess the impact of the scheme against this scenario. Unfortunately this report is much less comprehensive than the LMVR and there is important information missing, as discussed below.

Do-Minimum

In order to construct realistic Do-Minimum scenarios for the future years which are modelled, 2024 and 2039, assumptions need to be made about changes in the highway network, background traffic growth and specific land use developments in the area.

The highway schemes included in the 2024 model appear reasonable, although there must be some doubt about whether they will all be built by 2024. It does not appear that any further projects are assumed between 2024 and 2039.

The approach to assessing traffic growth was to apply TEMPRO, adjusted to be consistent with the National Trip End Model. TEMPRO is a government database that makes estimates of population and employment at a Local Authority District level for 5 year time intervals into the future, which are then converted into journeys. Specific proposed developments were then added, based on GMSF and the development plans for High Peak and Cheshire East. Finally, the resulting trip totals were factored down to match TEMPRO. As a result, total travel – at a District level – is consistent with national forecasts, but the distribution of development is based on the Local Authorities’ plans. Constraining the forecasts to TEMPRO is a government requirement, designed to stop multiple Local Authorities assuming that they each will gain a disproportionate share of a finite level of assumed national employment and population growth. Clearly, the case for a project can be enhanced if it is assumed that a high proportion of development in an area is located in places that will benefit from that project.

The report does not specify what developments are included in the model, or what proportion of them are expected to have occurred in each of the 2024 and 2039 time horizons.  This is said to be included in the Uncertainty Log. However a flow difference diagram in the SOBC implies high levels of development at High Lane, Woodford and near M25 Junction 25 by 2024, all of which are close to the Link Road. Given the responses to the GMSF public consultation, these developments are highly controversial and Andy Burnham has committed that “the redrafted spatial framework will aim to make the most of Greater Manchester’s brownfield sites and reduce the impact on greenbelt” (3rd November 2017).

Tables 5.1 and 5.2 set out the forecast number of trips in each time period for 2024 and 2039, split by journey purpose and vehicle type. Oddly, the tables predict a reduction in the total number of journeys between the two years, of 5.5% in the morning peak, 6.1% in the evening peak and 3.0% in the inter peak. Only Light Goods Vehicle trips are assumed to increase. This could still imply an increase in total traffic if journey lengths are assumed to rise.  However, prima facie, if traffic is predicted to decrease, it raises questions about the need for the project. An explanation might be that development trips have been excluded and the numbers do not represent the true number of trips modelled. This needs to be clarified.

Forecasting Scenarios

The report refers to multiple scenarios having been assessed:

  • Section 5.1 refers to “forecast year scenarios for the preferred scheme and lower cost alternative”
  • Section 5.3 states “data was collated in a format appropriate for the purposes of the uncertainty log and alternative scenario creation”, and
  • Tables 5.1 and 5.2 contain the column heading above the trip data “Do-Minimum Core and Do-Something Core”, implying there are other scenarios, as indicated in the text above the table

However, no information is provided about any scenario except the preferred scheme and a single pattern of development in either the Forecasting Report or the SOBC. Good practice in forecasting is to assess a range of possible scenarios in order to examine whether the case for the project is robust. It seems that this may have been done, but the results have not been shared. Similarly the government requires a lower cost alternative to be assessed to check if there is a cheaper way of meeting the project objectives. Again the results have not been shared. This could raise the suspicion that Councillors are being bounced into a decision without full information.

In relation to the scenario which is reported, the following observations are relevant.

In Section 1.2.6 of the Business Case Main Volume, reference is made to complementary measures in accordance with SEMMMS strategy to make efficient use of road space where traffic flows are reduced and mitigation measures to reduce the effects on communities with extra traffic. There is no reference to complementary measures in the Forecasting Report, implying they have not been modelled. The following reference is made to mitigation measures: “The mitigation measures assumed in this tranche of modelling include improvements to the junction of the A6 with Windlehurst Lane to provide additional capacity for traffic to/from Windlehurst Lane”. Whether this would be regarded as an ”improvement” by local residents is open to question.

As noted above, the A6 outside Greater Manchester and areas south of a line broadly between Adlington and Macclesfield have not been included in the economic appraisal. Additionally, traffic impacts in Macclesfield may not have been simulated in detail.

The results presented include changes in traffic flow on each road crossing three screenlines. Screenline 1 intercepts north south traffic on a line which runs from just east of Princess Parkway, along the north side of M60 to Bredbury and then across A6017 and A560 north and east of Bredbury. As might be expected it shows an increase in traffic on M60 and a mix of increases and decreases on other roads.

Screenline 2 assesses flows to the east of the scheme from Bredbury to High Lane. The screenline appears to be poorly designed as Figure 6.1 (page 243 of Appendices 6-9) shows that it intersects the A627 north- south flow at two points : A627 at Otterspool Road and A626 near Seventeen Windows, where it is combined with Marple – Stockport traffic for a short distance. The A626 at this point is wrongly called Buxton Road (it is actually Marple Road). This throws some doubt on whether Figure 6.1 shows the actual screenline locations. Assuming that it is correct, Table 6.5 (page 246 of Appendices 6-9) shows an increase in traffic on the A6 in High Lane, a reduction in traffic on Windlehurst Lane (calling into question why an improvement is needed at its junction with A6) and a substantial reduction on A627. If the reduction in traffic on A627 is removed from the flow on the short section of road shared with A626, the data implies that the east-west flow through Marple would increase as a result of the project. This is shown below

 

 

 

Link Eastbound Flow Change Westbound Flow Change
2024 2039 2024 2039
AM Peak
A627 Otterspool -199 -228 -357 -223
A626 Seventeen Windows -188 -213 -28 -73
A626 Dan Bank (implied) +11 +15 +329 +150
Interpeak
A627 Otterspool -101 -108 -329 -223
A626 Seventeen Windows -169 -266 -67 -73
A626 Dan Bank (implied) -68 -158 +262 +150
PM Peak
A627 Otterspool -244 -355 -327 -223
A626 Seventeen Windows -209 -76 -69 -73
A626 Dan Bank (implied) +35 +279 +258 +150

 

The table indicates that an increase in flow is probable in both years and in both directions in all time periods except eastbound in the inter-peak. It should be noted that the table in the Forecasting Report shows identical westbound flows at all screenline crossing points in all three periods in 2039. This is almost certainly an error in the report.

Figure 6.1 shows Screenline 3 running from Bredbury to the east of Marple and New Mills, while Table 6.6 shows it running from Bredbury to the south of Stockport to the A34. The latter is a more reasonable location, although the exact location of the screenline crossing points is uncertain. The table labels directions Eastbound and Westbound, when they are largely Northbound and Southbound, making interpretation difficult. However the table shows a substantial increase in traffic on A626 Offerton Lane, except in one direction in the PM peak. Combined with the increase in traffic implied further east on A626, this implies a significant worsening of congestion between Marple and Stockport. However, there is a reduction in traffic in the A6 corridor.

The poor labelling, probable error in Table 6.5 and inaccurate Figure 6.1 reflect badly on WSP PB’s Quality Assurance processes and the Forecasting Report should be revised to correct them.

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