It is partly discussed in teh link from. the Telegraph, but we had the same claims in Portsmouth, but they were roundly dismissed as poor statistical analysis.
Most cases when you include the minor accidents will show a decrease. When these were included in Portsmouth the reality was a 20% reduction.
What was also interesting was that in some areas of Portsmouth, it was shown that due to lack of enforcement, some roads saw an increase in the speed along them!
Part of the problem was thought be a difference in perception.
Whereas pedestrians assumed the traffic would be slower and hence safer, the endemic speeding meant that the opposite