Sir - Several times lately, I have walked past the queue of traffic waiting for temporary traffic lights to change at the Magdalen Bridge roadworks.

Each time there have been several buses, coaches and lorries held up in the queue, and none of their drivers has had the sense to turn off the engine.

Except for buses equipped with particulate traps, diesel engines ticking over pollute the air with particulates that degrade air quality and affect our health. The driver of any vehicle caught at the front of this particular queue during busy periods knows that they have at least five minutes to wait before they can proceed, so why do so few drivers shut down their engines?

As the sunlit days of spring and summer approach, when photochemical reactions convert vehicle emissions into the poor air quality for which central Oxford has become renowned, why can't the bus companies serving the city together instruct their drivers to switch off when held in traffic or picking up passengers in the city centre. In Switzerland, this is standard practice for buses at all times of year, even on rural routes; I have seen drivers routinely switch off engines when standing even for a minute or two. Why not here too?

Robin Gill, Headington