New roads create new traffic

When a new road is built, new traffic will divert onto it. Many people may make new trips they would otherwise not make, and will travel longer distances just because of the presence of the new road. This effect is known as ‘induced traffic’. 

Traffic jam

Induced traffic means that the predicted congestion benefits of a new road are often quickly eroded. Traffic levels on bypassed roads can also rise faster than expected due to induced traffic, all of which means the hoped-for benefits of a new road can evaporate very quickly.

The phenomenon of induced traffic has been observed by transport professionals repeatedly since 1925! And recent authoritative reviews have confirmed that induced traffic is still beating forecasts on new roads across the country.

Induced traffic distorts spending priorities

Traffic jam

By not forecasting traffic levels properly and not fully taking into account induced traffic, the benefits and costs of a new road will not be accurately calculated, which can lead to big mistakes being made with public money.

  • During the planning stages of a road scheme, if induced traffic is not included properly the full environmental impact of the scheme - for instance, carbon and noise levels - will be underestimated.
  • By predicting a longer period of relief from congestion, underestimating induced traffic will further distort the benefit-cost ratio so that decision makers will have false expectations of the economic returns of a road. 
  • Local people will also have too high expectations for a road scheme if induced traffic is not included properly in forecasts. 

And even if induced traffic is forecast correctly, it will still cause damaging environmental impacts and extra congestion that make building big new roads more pointless than ever.

Expert studies

The most authoritative studies into induced traffic are the 1994 SACTRA report 'Trunk Roads and the Generation of Traffic', and the 2006 report 'Beyond Transport Infrastructure' by independent consultants for the Countryside Agency and CPRE.

SACTRA report 1994 - Trunk Roads and the Generation of Traffic

Cover of 1994 SACTRA report on traffic generation

SACTRA was an independent panel of experts set up to advise the Department for Transport on the impacts of road building. Professor Phil Goodwin, one of the lead authors of the report, said:

"The average traffic flow on 151 improved roads was 10.4% higher than forecasts that omitted induced traffic and 16.4% higher than forecast on 85 alternative routes that improvements had been intended to relieve. In a dozen more detailed case studies the measured increase in traffic ranged from 9% to 44% in the short run and 20% to 178% in the longer run. This fitted in with other evidence on elasticities and aggregate data."

The conclusion of the report was:

"An average road improvement, for which traffic growth due to all other factors is forecast correctly, will see an additional [i.e. induced] 10% of base traffic in the short term and 20% in the long term."

The DfT accepted the report’s conclusions.  However, are the lessons being learned, and are traffic forecasts still inaccurate?

Beyond Transport Infrastructure: Lessons for the future from recent road projects

The Newbury bypass passes over a canal

In 2006, consultants Lilli Matson, Ian Taylor, Lynn Sloman and John Elliott examined three major road schemes in detail for an important report for the Countryside Agency and CPRE. 

The schemes included the infamous A34 Newbury Bypass which attracted mass protests in 1996. They also examined 10 other schemes built since the publication of the 1994 SACTRA report and used data supplied by the Highways Agency's own Post Opening Project Evaluation (POPE) studies.

In the case of Newbury, the report showed that traffic levels predicted for 2010 in Newbury were already reached by 2003 – and that traffic had increased by almost 50% in that period. New development around the road was partially to blame for the increases.  
In the other case studies the report concluded that:

"Traffic growth on the routes considered was higher than forecast, sometimes quite dramatically so."


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