National Highways' formal appraisal of its flagship M25 Junction 10 scheme significantly understated the impact and cost of delays during construction, a Highways investigation suggests.
As Highways has reported, the £317m scheme, which began last year and is due to run to the summer of 2025, has caused major jams, with nearly two million vehicle hours of delay last year. This figure is based on delays to vehicles remaining on the mainline.
Using values in official WebTAG guidance and National Highways’ data, Highways has calculated the likely cost of the delays to all vehicles to be around £30m a year.
However, the economic evaluation for the scheme’s planning application used an estimate of £11m for the total cost of delays caused by the scheme's construction. This was half the amount provided in an earlier estimate of £23m and just a quarter of a £45m forecast produced the same year.
The figure of £23m (at 2010 prices) appears in a National Highways (known then as Highways England) November 2017 Scheme Assessment Report and was based mainly on reducing speed limits to 50mph on the M25 and A3 mainline in the immediate vicinity of the junction.
This was despite the report assessing that construction 'would involve a complex programme of traffic management on the live highways, including: reduced speed limits, narrow lanes, lane closures, overnight closures of the road with diversions'.
In 2017 National Highways decided to take forward Option 14
However, the scheme’s April 2020 Combined Modelling and Appraisal Report, obtained under freedom of information laws, calculated a cost to the economy of just £11m, based on the same assumptions and methodology.
The appraisal report gave the scheme a benefit cost ratio (BCR) of 2.22. The Planning Inspectorate (PINS) recommendation report, sent to the DfT in October 2020, cites this BCR.
However, the scheme’s traffic management plan (TMP) predicted that there would be lengthy delays for vehicles using the junction to change from the M25 to the A3 or vice versa and estimated the cost at £15m a year for each of three years, therefore totalling £45m.
Asked to explain why the appraisal report had the lower figure, a National Highways spokesperson told Highways that the Combined Modelling and Appraisal Report was 'based on the best knowledge at the time, in this case 2020'.
It added that the TMP is 'a detailed plan once the construction methods are fully known and takes into account the method, safety and our customers into its assumptions. The value presented in the Traffic Management Plan is based on more up to date information’.
However, the TMP states: ‘Construction period is January 2021 to December 2023’ - meaning it must have been written no later than 2020.
The first iteration of the TMP is dated just four days after the PINS report in October 2020, which suggests that the estimates within it were generated while PINS was considering the case - raising questions about why the £45m figure was not made available.
The TMP was produced well ahead of the decision in May 2022, by the then transport secretary, Grant Shapps, to grant the scheme’s Development Consent Order (DCO), based on PINS’ recommendation.
The final iteration of the TMP was published the same month. It was a requirement of the DCO and the published version is stated to have been produced ‘for discharge of requirement’.
Highways asked the DfT whether it was aware when the DCO was granted that National Highways was predicting a much greater economic impact of the delays than it used in its application. It refused to answer.
It is also clear that National Highways was aware at the time of the appraisal report that lane closures on slip roads would impact traffic flow and the TMP assumed that lanes on the mainline would largely remain open.
The loss of lanes on slip roads and the junction roundabout has subsequently proved to be the major cause of delays, causing huge tailbacks onto the main carriageway. This suggests that a valid estimate of the impact of the scheme would have covered both types of journey.
National Highways told Highways that the increase in delays for vehicles remaining on the carriageway last summer ‘reflects the introduction of traffic management on the sensitive slip roads leading on/off J10’.
Becca Lush of campaign group Transport Action Network (TAN), who obtained the appraisal report, told Highways: ‘TAN is not surprised that National Highways is underestimating the costs of its major road projects, including the construction delays.
‘Downplaying costs and exaggerating benefits seems to be endemic in road appraisal. It hoodwinks ministers into approving expensive and damaging projects which don't deliver value for money. It's time there was a light shone on this rather murky process, even more so when money is tight.’
Highways also asked National Highways whether it expected the extent of the disruption and whether it has conducted any kind of lessons learned exercise.
The company said that it anticipated a ‘notable’ impact on daily traffic flow and that it had sought to ‘minimise’ disruption.
