But it also seems to boost the number of short journeys taken by bus, which might otherwise have been cycled or walked, the findings show.

The researchers wanted to assess the public health impact of giving teens in London free bus travel. The scheme was introduced for 12 to 16 year olds in 2005, and for 17 year olds in 2006.

They therefore used data from the London Area Transport Survey and London Travel Demand Surveys to calculate the number of journeys made in London -- as well as distance and principal mode of travel -- before (2001-4) and after (2005-9) the scheme was introduced.

And they looked at official data on traffic injuries and hospital admissions to see if the scheme had any noticeable effects on personal safety.

The analysis showed that the proportion of short journeys teens took by bus doubled from 2% to 5%, although the overall number of journeys they took did not increase.

The number of short trips walked also fell in tandem with an increase in this length of journey taken by bus, although there was no appreciable impact on total distance walked.

But there was clear evidence of a fall in the number of short journeys cycled and in distances cycled by young people, although this mode of travel was not hugely popular among this age group before the introduction of the scheme.

Rates of road traffic casualties had started falling before the introduction of the scheme, and continued to fall afterwards, but at a greater rate in young people, largely among passengers and cyclists. Pedestrian casualty rates remained the same.

Hospital admission rates for assaults had been rising among teens before 2005, but were higher among this age group after the scheme's introduction.

The number of daily car journeys taken by young people and adults fell, and the average distance travelled by car also shrank, suggesting that free bus travel prompts a shift away from car use and may therefore be a greener option.

There didn't seem to be any fall in the use of buses by older people after the scheme's introduction either.

"The findings suggest, unsurprisingly, a good uptake in use of buses for fulfilling travel needs, including for short journeys," write the authors.

"One disadvantage appears to be some reduction in the proportion of short trips by walking, and in the (already) low level of cycling; these might be detrimental to the establishment of future travel habits bringing regular physical activity," they suggest.

"On the other hand, the increase in the use of public transport may help to establish travel behaviour for later life that entails some physical activity, as well as helping to reduce car use," they say.