UK politicians, national and local, tend to like investing in roads. The Treasury considers the Ministry of Transport’s approach to economic analysis to be sound and is therefore prepared to provide substantial funds. The ministry is happy to accept them, as are the civil engineering contractors who benefit from them.

The most recent result is a £ 27.4 billion investment program designed to maintain and expand England’s motorways and A-roads over five years. Part of this will involve the creation of so-called smart highways, where the emergency lane is converted into an additional lane for traffic and electronic message boards broadcast information to control driver speed and handle incidents. .

This innovation has raised safety concerns, with some worrying about the risk of collisions in the event of a breakdown. Elsewhere, the decision to invest in 4,000 kilometers of road has been criticized for underestimating the associated carbon emissions.

Inside an intelligent highway control center.
Highways Agency / Flickr, CC BY

The government’s defense is that expanding the road network helps reduce traffic congestion and spurs economic growth. I decided to examine these claims by analyzing the outcome of extending a section of the M25 to a smart highway in 2014.

For each of these programs to be given the green light, a cost-benefit analysis must confirm that they offer good value for money. This is usually provided by transport consultants who operate complex models that require them to estimate many different parameters. Model makers naturally wish to please their customers with their best estimate of the quality / price ratio. But the resulting optimism bias leads politicians to overstate the benefits of adding new road capacity. On this point, the M25 case study shows that the public is misled.

A smart decision?

Highways England, a company owned by the Department for Transport and responsible for the country’s motorways, has published detailed traffic monitoring reports for the first three years after the opening of an intelligent motorway system between Junctions 23 and 27 of London’s M25 orbital route.

The road was widened from three to four lanes in each direction. If traffic accelerated a year after opening, this advantage was lost in the second year thanks to the increase in traffic volume, up 16% against 7% for other highways in the region.

The road investment is supposed to benefit the economy by reducing travel time by precious minutes. Traffic models are used to estimate the likely time savings to justify each investment. The model used in the case of the M25 provided for substantial travel time savings worth over £ 400million for people traveling for business – both cars and good vehicles.

Local road users, whether commuters or short trips, have also seen less time savings. But these were almost entirely offset by rising fuel costs. Indeed, these local drivers turned to the highway where there was less traffic to save a few minutes on their journey. Ultimately, however, they ended up traveling a greater distance away from more direct routes.

The M25 traffic model used to justify the smart highway investment significantly underestimated this increase in traffic volume, while overestimating the average speed increase for most drivers, estimated to be around 10 km per time. The benefit-cost ratio was estimated at 2.9, or £ 2.90 of economic benefit for every £ 1 invested. Since the travel time savings did not last beyond the first year after opening, the actual benefit-cost ratio was much lower.

Two four-lane highways side by side.
This section of the M25 has lost its emergency lane, creating four traffic lanes.
Philafrenzy / Wikipedia, CC BY-SA

While the widening of the motorway was intended to benefit the economy by allowing people traveling for their employer to save time and thus be more productive, the additional road capacity was largely absorbed by an increase in road users. the local route. Any economic benefit they could have gained by saving a few minutes of travel time was offset by higher fuel costs.

The M25 was created to allow long distance traffic – for example, from Channel ports to the Midlands – bypassing central London. Although local road users inevitably take advantage of the freeway for short trips, sometimes referred to as “junctions at the junction,” these reduce economic benefits and increase carbon emissions and air pollution.

England’s motorways and main roads are most heavily used around urban areas, where local and long-distance traffic compete for road space. This is where the government’s new road investments are concentrated. The current investment program includes ten smart highway projects, with an average benefit-cost ratio of 2.4. It sounds extremely optimistic in light of what happened with the M25.

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