Ask Leo

I recently stopped travelling by bus to work and started cycling. I felt good about helping the environment. Then a friend suggested I was making things worse. His logic was that the amount of diesel I was saving by not travelling on a bus was tiny. By cycling in bus lanes, making buses behind me slow down and drive along in an inefficient manner, I was causing them to use far more extra fuel than I was saving. Is he right?

Jeff Bloom, London

On yer bike, Jeff. This seems to be the advice of those who have studied this conundrum. While there is general agreement that by slowing down a bus with your two-wheeled dawdling you are indeed causing it to chuck out more emissions than if it was cruising along unimpeded, you need to view the bigger picture.

Professor Frank Kelly, who leads the environmental research group at King's College London, which specialises in studying the impacts of urban air pollution, says that this dilemma "illustrates well how we can sometimes lose perspective". First, buses are getting "pretty clever", he says, and, with "engine management and injection timing" they can optimise their efficiency during their typical stop-start journeys. Second, and more importantly, the more people cycle, the less need there is for other road traffic. This is the greater good cyclists need to consider.

Richard George, roads and climate campaigner at the Campaign for Better Transport, agrees: "By cycling you are encouraging yet more people to cycle. All the studies show that cyclists feel safer when surrounded by other cyclists. And by encouraging more people out of cars and on to bikes you are reducing traffic levels and therefore allowing buses to travel more efficiently and faster." The real problem, says George, is not the plodding cyclist but the illegally parked cars and vans that stop in a bus lane, throw on their hazard lights, and nip into the adjacent shop.

Kelly adds that it's best just to avoid bus lanes altogether by getting hold of a free cycle map from your local council. Or, he says, "just cycle a bit quicker".