Official analysis of existing schemes has shown that bypasses don't reduce traffic. Instead, they encourage more people to drive and often just move the problem a few miles away.
A series of detailed studies, carried out by the Highways Agency to see the effect of new roads after they have opened, found that there are many reasons why bypasses don't work.
Bypasses don't always take traffic away from the old route
A6 Alvaston Bypass: did not attract as many vehicles as expected because the Highways Agency did not tackle a nearby junction (A6/A511 Raynesway), which made the bypass less attractive than the old route. Traffic on the old route was 225% higher than expected, and down 29% on the bypass
A1(M) Wetherby to Walshford: The Highways Agency expected people to use the bypass. But drivers preferred the older, less trafficked route, which has 1250% more traffic on than expected
Bypasses just shift the problem a few miles down the road
A6 Great Glen Bypass: when the Highways Agency bypassed Great Glen, people in the surrounding towns complained about the traffic. Now residents of nearby Kibworth Harcourt want a bypass, and people in Newton Harcourt complain that the village is too congested and overrun with speeding traffic
A650 Bingley Relief Road: local residents complained that the relief road wasn’t getting them to work any quicker, because while it had reduced congestion in Bingley, the traffic jams had just moved to Saltaire
Bypasses often pull in traffic from other parts of the region
A43 Syresham & Silverstone Bypass: The bypass attracted traffic off the M40 and M1 (10% decrease north of junction 10 of the M40). Syresham had 40% more traffic on the bypass and Silverston had 26%
A34 Newbury Bypass: The bypass 'unlocked' the A34, attracting people from across the South of England. Traffic on the bypass was almost 50% higher than predicted. Read more about the failure of the Newbury Bypass in 'Beyond Transport Infrastructure' - a 2006 study looking at lessons to be learned from recent road projects.
Bypasses can remove trade from town centres
A27 Polegate Bypass: Local businesses in Polegate were losing so much money because people were using the bypass and therefore not coming in to Polegate, that they're calling for a sign on the bypass telling people not to use it!
A6 Great Glen Bypass: A number of local businesses in Great Glen have had to close because the bypass has reduced the volume of passing trade.
Bypasses often don't reduce journey times
A63 Selby Bypass: The route was supposed to substantially improve journey times, but 36% more traffic used the bypass than was expected, causing congestion. Journey time savings were 44% lower than expected. The scheme was also 60% more expensive than expected.
The Highways Agency has trouble modelling traffic growth accurately
A6 Clapham Bypass: The Highways Agency modellers predicted traffic would grow twice as fast as it actually did. There is therefore less traffic on the old route (16%), bypass (33%) and across the corridor, than they expected. They overstated the case for the road, which, if they had used the right data, might not have been needed in the first place
A6 Great Glen Bypass: Modellers did not take account of the A14 as a long-distance strategic corridor. This attracted more traffic away from the A6 – up to one-quarter fewer cars than were expected to used the new bypass. If this had been taken account of in the first place there may have been no need to build the bypass
- Government archive page showing the Highways Agency 2009 reports on 'Post Opening Project Evaluation'
Our summary of these results: The Highways Agency's Billion Pound Traffic Gamble, January 2010
The full set of 2009 meta-analysis documents released to us by the Highways Agency: