Smart motorway cash outcomes 'could be negative'


Spending on smart motorways fails to deliver the economic benefits predicted because local road users take up the increased capacity, negating the intended benefits, according to a former chief scientist at the Department for Transport.

Dr David Metz, honorary professor at the University College London Centre for Transport Studies, put forward this analysis in a submission to the Transport Select Committee’s inquiry into ‘the roll-out and safety of smart motorways’.


He told MPs ‘My conclusion is that there is much less benefit in adding capacity than has been forecast on account of the diversion of local trips onto the motorway, which negates the projected benefits to long distance users.’

Dr Metz said he had carried out a detailed analysis of the outcome of a smart motorway investment and compared this against the forecast benefits predicted by traffic and economic modelling commissioned by Highways England.

He pointed out that under the Smart Motorway Programme the M25 was widened in 2015 from three to four lanes in each direction between junctions 23 and 27, ‘with detailed monitoring of traffic flows before and for the first three years after opening’.

He noted: ‘Some initial speeding up of traffic was observed at the first year after opening, but this gain was lost by year two on account of the increase in traffic volume. By the third year after opening the growth of average daily traffic of 16% was substantially greater than on major roads in the region (7%) and greater than forecast in the transport modelling used to justify the investment.’

Dr Metz added that while modelling had predicted a significant increase in traffic speed, which generated travel time savings that contributed to a benefit-cost ratio (BCR) of 2.9, representing high value for money according to the Department for Transport’s categorisation, ‘these time savings were not observed beyond the first year’.

He added: ‘The model forecast substantial time saving benefits to business users. There were also time saving benefits projected for local users, commuters and others, but these were almost entirely offset by increased vehicle operating costs, reflecting diversion to somewhat longer routes to take advantage of the faster travel possible on the widened motorway.’

He suggested that it is likely that much of the additional traffic arose from local users aided by electronic mapping aids rerouting to take advantage of shorter journey times via the motorway, while incurring greater fuel costs.

He noted: ‘Such longer trips give rise to increased negative externalities, including carbon emissions, air pollutants etc, which are set against the user benefits. The outturn BCR must therefore be quite small, possibly even negative.’

Dr Metz suggested that his findings regarding the M25 are ‘likely to be representative of much planned investment in new capacity on the Strategic Road Network, which comes under greatest stress in or near urban centres where local traffic competes for carriageway with the long-distance users for whose benefit the investment is primarily intended’.

He stated: ‘Highways England includes 10 smart motorway investments in its current five-year investment programme with a forecast average BCR of 2.4. This seems very optimistic.’

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