True cost of traffic delays from building major roads

May 9, 2025

There is concern that National Highways is routinely underestimating the impact and cost of delays caused by the construction of new road schemes. It does this to skew the business case and secure approval.

When planning “enhancement” works on its network, National Highways is obliged, as with any other major project that spends public money, to draw up business cases and benefit cost ratios (BCR). Given that benefits claimed for such schemes mainly come from the time saved by road users, the costs of the disruption to road users during construction work should also be taken into account.

The Government’s Transport Analysis Guidance (TAG) states that these costs should be included in calculations “where they are likely to be significant”^[1].

The main costs to motorists during construction are time wasted and extra fuel used in traffic jams and following often lengthy diversions. The two key factors that influence this are how long the scheme will take to construct and how traffic, including construction traffic, is managed during this time.

M25 Junction 10

National Highways’ huge scheme at Junction 10 of the M25 (where the M25 meets the A3 at Wisley) began in 2022 and was due to end in Summer 2025. However, in early 2025^[2] the company admitted that it would finish at least nine months late – in Spring 2026.

In 2024, Highways Magazine revealed that National Highways had rigged its estimates of the cost of construction delays when working out the scheme’s BCR^[3]. A 2017 appraisal noted that traffic management would involve reduced speed limits, narrow lanes, lane closures and overnight road closures with diversions^[4]. However, rather than taking all this into account, it based its estimate on the time lost by slowing vehicles down to 50mph.

This produced a cost of £23m at 2010 prices, but when the company calculated the BCR in 2020, this cost had somehow dropped to just £11m – but without any explanation^[5].

After the scheme secured planning consent, a 2022 Traffic Management Plan (TMP) gave the estimated cost of traffic delays caused by the scheme at £45m, based on a three-year construction time (also in 2010 prices)^[6]. This is a 300% increase from the £11m estimated in 2020 when National Highways was trying to get planning approval.

Since construction started, disruption to the M25 and A3 has been far worse than predicted, with lengthy jams on both roads, while multiple closures over entire weekends have attracted national media attention^[7] after causing chaos and costing drivers time and fuel.

In 2024 Highways Magazine published National Highways data released under freedom of information laws showing that vehicles were delayed by more than a million hours in the last six months of 2023 alone^[8]. Using the same methodology as National Highways uses to estimate journey time savings, the magazine put the total cost of the disruption caused by the scheme at around £100m (2010 prices) over a 3-year construction period^[9].

One business that has been particularly badly affected is the Royal Horticultural Society, which runs the nearby Wisley Gardens. In January 2025 it asked the government to compensate it for what it says will eventually be £11m in lost income^[10]. Ministers refused the request, claiming that the impact of the scheme on RHS Garden Wisley was “considered from the project’s early stages in 2016” and that National Highways had “incorporated mitigation measures where possible”^[11]. In fact, the main mitigation measure in the TMP was to publicise disruption.

It is also likely that National Highways did not include this loss to a local business in its various estimations of the cost of the construction works which purely focused on time delays to motorists.

Other schemes

Business cases and appraisals for other schemes are also opaque when it comes to estimating the cost of traffic delays and additional vehicle operating costs. National Highways often calculates this as around 1% of the projected benefits.

One highly experienced transport planning consultant told Transport Action Network: “National Highways doesn’t care about understanding the real impact on motorists. It’s not in its interest to do so.”

Post-scheme reality checks

National Highways doesn’t check its estimates against reality either, despite regularly publishing Post Opening Project Evaluation (POPEs) of its major schemes^[12]. These POPE reports look in detail at the various benefits and costs of a scheme, including actual journey time savings and construction costs, to see if the scheme achieved what it was supposed to.

However, despite usually having actual journey time data, National Highways states that the evaluation of delay and vehicle operating costs during construction are “not evaluated” in POPEs, instead stating that they are “assumed as forecast”^[13]. Without knowing the cost of delays to the economy, any evaluation of whether a scheme provided value for money is meaningless.

The Lower Thames Crossing

The Lower Thames Crossing, which was given development consent in March 2025, will be roughly 14.5 miles of new smart motorway passing through a tunnel linking Kent with Essex. It is the most expensive road scheme in the UK, with an unusually long 7-year construction period.

The Planning Inspectorate’s report on the scheme^[14] found that, as with any large road scheme, increased congestion on the local road network during construction is inevitable and could last years in some places. The Inspectorate largely dismissed the concerns of local highway authorities and businesses, but also said that much of the detailed traffic management still needed to be worked out. This seems a rather cavalier approach to what will be the biggest road scheme for a generation causing widespread and lengthy disruptions. Unfortunately the government agreed, saying that construction disruption had been “mitigated as far as practicably possible in accordance with established best practices” despite the detail not having been sorted out^[15].

For the most part, delays caused by the scheme would result from construction and worker vehicles using the existing road network.

One hotspot is the busy Asda Roundabout, near the A13 and A1089 junction in Thurrock, where concerns have been raised by the local council and the Port of Tilbury^[16]. National Highways claims to have put in measures to manage construction traffic impacts, but Thurrock Council remained concerned that the measures were either unenforceable or relied on the good will of HGV drivers^[17]. .

In 2022 National Highways estimated that the cost of delays caused by the construction of the scheme would be £131 million at 2010 prices^[18], which equates to around £200 million today.

But, based on National Highways’ significant underestimate of the impact of the M25 Junction 10 scheme, and the LTC’s proximity to critical ports at London Gateway and Tilbury and interaction with the M25 at Junction 29, the cost of congestion caused by the 7-year construction of the LTC could easily top £1bn on time delays alone.

What’s the solution?

National Highways needs to take a more transparent and systematic approach to predicting and mitigating the costs to road users of building new road schemes. It also needs to assess these costs as they happen and publish them in POPE reports once the scheme is open, or it is destined to keep underestimating costs. Not only is this allowing it to understate the true costs of road building, meaning that poor value schemes are more likely to be approved, but it is also missing an opportunity to learn and to save the taxpayer money.

The Netherlands, for example, employs advanced traffic simulation models to predict and mitigate congestion during construction projects. This allows planners to identify potential bottlenecks and estimate costs; to incentivise behaviour change to reduce congestion, including promoting alternative routes, adjusting travel times and encouraging public transport use; and to produce integrated traffic management plans, which combine simulation insights with real-time traffic management strategies.

However, for this to happen will require a complete overhaul of the way that National Highways operates with closer scrutiny of its figures during planning consent applications, and publicly available data after opening. Otherwise, change is unlikely to happen, business cases will continue to be over-optimistic, poor value road schemes will continue to be built, and the economy and the travelling public will continue to suffer.

Photo: Shutterstock.com

[1] TAG UNIT A1.3 User and Provider Impacts 10.1.1

[2] M25 Junction 10 improvement project forging ahead – National Highways

[3] Highways Magazine – In search of lost time: M25 delay estimates don’t add up

[4] HE551522-ATK-GEN-J10-RP-ZM-000054 6.3.2

[5] PW Integrated Template 14.5 & 14.6

[6] Microsoft Word – HE551522-BBA-HGN-WHL_AL_SCHME-PC-WM-000001_P06

[7] M25 to close AGAIN bringing more ‘unprecedented’ chaos after drivers’ weekends were ruined by 10-mile diversion | The Sun

[8] Highways Magazine – Exclusive: One million hours of delays in six months – M25 scheme impact revealed

[9] https://edition.pagesuite-professional.co.uk/html5/reader/production/default.aspx?pnum=21&edid=cadd6b60-9b63-43a5-8e51-bd105d3a06e1

[10] RHS calls for compensation as A3/M25 roadworks cause devastating financial impact on charity / RHS Gardening

[11] RHS Garden Wisley: Roadworks – Hansard – UK Parliament