Sir – The other day, as I cycled into the Broad from the western end, I found the whole area pervaded by noxious fumes.

I soon identified the source: it was one of those open-topped city tour buses.

I caught up with it at the traffic lights and had to wait there in a cloud of mephitic gases till the lights changed, trying not to breathe in. Can the city authorities not clamp down on this noisome nuisance?

Martin West, Oxford