Letters: Limits on speed

Sir: Has any research been done into the psychology of speed limits?

As far as I am able to examine my own mind-set, I seem to relate the first digit on a speed restriction sign to the similar section of my car's speedometer. In other words, in a 30 area I seem to have a subconscious tendency to ensure I am driving under 40mph, rather than under 30 as I should.

It then takes further action on my part, usually with others breathing up my tail, to adjust to the lower side of 30. If my mental processes are common, why are speed limits not set with a nine as the second digit rather than a zero, ie 29, 39, 49, rather than 30, 40, and 50 respectively.

HARRY PAYNE

Southwell, Nottinghamshire