...or why unrepresentative data is not reliable.
If you don't have a TomTom device in your vehicle then you may be surprised that if you live in one of the places recently researched by Atkins for the their Evaluation of 20mph limits then it doesn't matter how much you keep within the speed limit your considerate driving was completely ignored. And you wouldn't be alone. The report estimated that the actual number of drivers who were also ignored were in the 97% of vehicles who's speed they never measured.
Atkins and DfT decided that in order to gather information on vehicle speeds in 20mph streets in 12 case study 20mph areas and compare them with 3 other 30mph areas then they would use data captured from high end TomTom SatNavs or vehicles with in-built TomTom in-car devices. This is termed Floating Car Data (FCD).
You may therefore wonder just how accurate and representative their statistics and research were if it only included 3% of vehicles. We did as well, so we did some of our own research.