Anyone who lives in south-east or east London know that how well traffic flows through the Blackwall Tunnel has a huge effect on our day-to-day lives. This doesn’t just affect people who drive to work: when the tunnel is closed a 20-minute bus journey can take an hour or more on a bad day.

So you might feel building a new tunnel is a reasonable response. After all, more room for all those that want to cross the Thames can’t hurt?

However studies show that building new roads is likely to increase the overall amount of traffic in the area. This is particularly true in places where demand for those roads is very high and the existing roads are operating close to capacity – sound familiar to you?

This extra traffic attracted to the area by new roads is known as ‘induced traffic’. Very often, new roads attract traffic far in excess of estimates. When these roads are planned, the benefits are overstated, as induced traffic isn’t included in the original case for building the road.

If it were a simple matter of moving traffic from the old infrastructure to the new infrastructure then the impact wouldn’t be so bad, but reviews of bypasses built around towns show that while the new roads show more traffic than expected, so do the old roads.  This means that overall traffic in the area can be far higher than before the new road was provided.

In 1996, a study into the phenomenon of induced traffic was carried out by an independent group of experts at the request of the Department for Transport. It found that where induced traffic was not included in the original estimates for new roads, then a year after opening, on average they showed 10% more traffic than the estimates suggested.

Meanwhile the old roads that should have benefited from less congestion showed a bigger rise: 16% more traffic than estimated used the old road over the same time period.

This overall increase in traffic means local residents will bear the burden of increased traffic – increased pollution, increased risk of traffic accidents, increased noise. It also means new developments may never achieve their expected potential.

Why might this happen? Reasons include…

• New developments such as supermarkets may appear on the road, generating extra traffic.
• An improved road might tempt commuters back into their cars, causing more traffic.
• Drivers might change the times they travel – perhaps to move back into the rush hour.
• If the new road offers a quicker route, a driver might decide to use that instead. So a lorry driver might decide driving via Blackwall/Silvertown is quicker than using the Dartford Tunnel, on far more suitable roads outside London.

Some of these effects will be felt very quickly after the new road is opened. But some may not appear for months or years.

When surveyed, drivers say that if congestion rises on a road they currently use, they’ll continue to use that road but adjust the time they make their journey, and if congestion drops on a road they don’t currently use they are more likely to start using the road. So empty roads attract drivers until congestion’s an issue there too, but full roads don’t deter people from getting in their car.  More roads may mean more cars, up to the point where conditions become too unpleasant for more people to drive on them.

At the No to Silvertown Tunnel campaign, we can already show that pollution near our homes already reaches dangerous levels – and that’s before any new traffic appears.

We think it very likely that instead of easing congestion, there will be an increased volume of traffic heading through Greenwich Peninsula, Charlton, Blackheath, Kidbrooke and Eltham, as well as through Poplar and the Royal Docks, and that both drivers and local residents will suffer as a result.