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 well-known and long-established effect is known as ‘induced traffic’.
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
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.
Read a summary of the long history of new roads generating new traffic by Professor Phil Goodwin:
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 was an independent panel of experts set up to advise the Department for Transport on the impacts of road building.
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?
Professor Phil Goodwin, one of the lead authors of the SACTRA 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."
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 ten 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."
John Elliot worked as a transport expert for the GLC in the 1980s and was asked to look at the long-term effects of increasing road capacity on key corridors in London.
Comparing roads that had been widened, such as the A40 Westway, with corridors where main roads had not seen significant capacity-building, it was clear that adding new road space had increased traffic far more than would be expected, and that this effect continued into surrounding streets and junctions.
His results are very well summarised in a presentation given at a public meeting in Greenwich in 2013 (see box).
Figure 2.4 in the presentation by John Elliot also shows the results of widening the Blackwall Tunnel in East London in the late 1960s.
The data for traffic flows at Blackwall, and at other Thames crossings nearby, show that when the capacity at Blackwall was doubled, traffic more than doubled in the first year of opening due to the immediate release of suppressed demand, and that this was accompanied by barely detectable levels of traffic relief at other bridges and tunnels.