Table 4

Observed and modelled changes in injury rates by severity with 95% CI based on Poisson distribution for counts and SEs from Elvik

Accident severityExponentObserved annual ratesExponential modelled ratesDifference between predicted post and observed pre-20 mph (%)Difference between predicted and observed post-20 mph (%)
EstimatePre-20 mphPost-20 mph% diffPredicted(speed+vol)% diffdiff% diff
Injury0.034 (0.014 to 0.054)897 (839 to 958)593 (546 to 642)−31 (−38 to –24)817 (762 to 875)−9 (−17 to 0)224 (150 to 298)27 (20 to 35)
Serious0.069 (0.045 to 0.093)119 (99 to 142)107 (99 to 142)−10 (−34 to 14)101 (82 to 123)−15 (−38 to 8)−6 (−36 to 24)−6 (−36 to 24)
Fatal0.069 (0.045 to 0.093)5.7 (2 to 13)4.7 (2 to 13)−18 (−143 to 108)4.9 (1.6 to 11.7)−14 (−135 to 107)0.2 (−7 to 8)4 (−145 to 154)
All casualties1022 (960 to 1087)705 (654 to 759)−31 (−38 to –24)923 (864 to 985)−10 (−18 to –2)218 (138 to 298)24 (16 to 31)
  • The casualty rates by severity are based on the ‘Accident Severity’ variable in the collisions dataset and averaged over implementation zone (online supplemental material 14).