Sir - Before asking for speed limits on the major roads into Oxford to be reduced to 20mph cyclists should think about one of the most dangerous situations that they can face.

If you are cycling at, say, a gentle 10mph and an overtaking vehicle is limited to 20mph its speed margin is only 10mph. It takes time to get past. If you are cycling faster the margin is less, overtaking will take longer and the distance covered before the overtaker can safely pull in is greater.

A quick guestimate of the distance travelled by a car going at 20mph while overtaking a bicycle doing 18mph (yes some do go this fast) is 150 ft. If other traffic is coming the other way at the same speed a clear road for 100 yards ahead is needed before starting. If another vehicle comes in the opposite direction unexpectedly the overtaking vehicle may have little alternative but to pull in towards the kerb, leaving you, the cyclist, in a literally very tight corner.

If the speed limit stays at 30mph, the margin is much greater. Overtaking can be completed in less time and requires much less distance. The faster speed limit results in very much less risk. As a quite different point, most vehicles can be driven in a higher gear at 30 mph than at 20. With the vehicle travelling in a higher gear for much of its journey, its engine turns more slowly. It emits less exhaust fumes. It makes little sense to restrict speeds so severely on the major roads into Oxford.

N. L. Gregory, Oxford