SafeSpeed wrote:
http://www.dft.gov.uk/stellent/groups/dft_rdsafety/documents/page/dft_rdsafety_030562.pdf
Contains:
"Those observed to be conforming consistently to the speed limit differed from those observed to consistently break the speed limit in several ways. They were older, drove lower annual mileages, were more likely to be female and were more likely to report having been accident involved in the previous three years. However, this difference reached statistical significance only in the case of age."
Which tends to confirm the widely-held suspicion that most of those who rarely exceed speed limits do so not as a deliberate choice but because they are nervous and timid drivers with little confidence in their own abilities.
It's also rare indeed to see another driver observing speed limits in the way one would be expected to on an IAM test. Those who rarely exceed 30s will often drive well below that speed even when safe to do a few mph faster, and also make little or no adjustment to their speed when passing into a higher speed limit.