That is the central finding of new research by Green and colleagues, to be presented at the Royal Economic Society’s 2015 annual conference later this month.

Professor Green said: ‘Our results suggest that the congestion charge has broadly changed people’s modes of travel and the number of trips made into central London, with a beneficial reduction in accident costs and lives lost.

‘What’s more, it has even reduced the probability of an accident for those that continue to commute by car.’

In 2003, in an effort to reduce congestion, London introduced a £5 charge for motor vehicles that enter central London. When initiated, the charge was hailed as a triumph of economics, forcing those contributing to congestion to pay an explicit price. The resulting reduction in traffic congestion confirmed predictions that the charge would change behaviour.

The introduction of the charge has implications for traffic accidents, related injuries and fatalities. Reduced congestion means fewer cars in central London and an expectation of fewer accidents. Yet, it also means higher travel speeds, which could increase the chance of an accident and the severity of accidents that do occur. The increase in speed may be particularly dangerous in central London where cars, cyclists and pedestrians share the road.

This study provides the first evidence on the effect of the congestion charge on the number of traffic accidents, injuries and fatalities and on the rate of these accidents. The researchers show that introducing the charge led to a substantial reduction in the number of accidents within central London. Moreover, the charge led to a significant decline in the rate of accidents per mile driven.

A simple comparison of before and after is uninformative. Such analysis may simply be observing a decline that would have happened without the charge. Thus, the researchers adopt a variety of control groups using the most populous 20 cities in Britain, not including London. They contrast the change in accidents in the congestion charge zone to that change over the same period in these other cities.

Their main result – that introducing the congestion charge reduced traffic accidents in central London by 30 a month, an enormous 40% reduction – is matched by similar reductions in those killed or seriously injured. Thus, in addition to saved travel time, the congestion charge saved accident expenses and lives.

While this represents an important public health finding, a key economic issue is the influence on the probability of accident and injury. A decline proportional to that in travel miles would indicate no change in this probability. The researchers therefore examine the rate of accidents – the number of accidents per million miles driven in the zone.

Again, they compare changes in this measure to other cities and find large reductions in the probability of experiencing an accident, injury or fatality in the zone. For example, the number of accidents per million miles averaged 12.4 prior to charge and the researchers estimate that this fell by 2.6 accidents as a result of the charge. This estimate indicates that the reduction in congestion brought a more than proportional decline in accidents, making it safer for those that drive.

This decline in accidents extends beyond the boundaries of the charge programme. Accidents did not increase in adjacent areas but decreased as fewer people drove through them to reach central London. The estimates also show that accidents and injuries were reduced in non-charged times (before 7am and after 6pm) and for exempt vehicles (largely bicycles, motorcycles, taxies and buses).

Bicycles in particular raised policy concerns as the charge resulted in more cycling into central London. The study finds a small initial increase in accidents involving cyclists, roughly one and a half a month up to 2005. Yet, by the end of 2006 this reversed, and cycling accidents and fatalities fell as result of the congestion charge.