The problem with buses

It's received wisdom that public transport causes less pollution than the use of private cars. To the pedestrian, child in a buggy or cyclist however, the opposite feels true. The bus roars past (bus drivers are always accelerating, braking or swerving to maximize the jerky effect) and we all know what to expect: a swirling fog of warm, particulate-laden, diesel exhaust through which we must hold our breath until we are clear. Bus exhaust outlets seem designed to do this deliberately, pointing down towards the road surface to ensure an even distribution of toxic gases at the breathing level. This is not how it has to be. Many continental cities have buses whose exhausts vent vertically. Granted the gases have to go somewhere and upper-storey windows over the street may have to stay closed during busy times. But the city of Florence solves this by using smaller, electric buses for the clogged inner city routes. Most big trucks have vertical exhausts. Why not buses, which spend so much of their journeys belching their way past pedestrians in narrow streets?