Table 1

Average annual road traffic casualty and collision rates by speed limit road segments citywide

Road typeSum (pre)Sum (post)Rate (pre)Rate (post)Rate (diff)Rate (% diff)Diff in diff (%)
Collisions
 20 mph existing streets354121118 (98 to 141)66 (51 to 84)−52 (−69 to –35)−44 (−57 to –33)−10 (−24 to 5)
 20 mph local and main streets1568550523 (479 to 570)301 (268 to 337)−222 (−258 to –186)−42 (−48 to –37)−8 (−18 to 2)
 30 mph802321267 (236 to 301)175 (150 to 203)−92 (−119 to –65)−35 (−43 to –26)REF
Casualties
 20 mph existing streets389139130 (108 to 154)76 (59 to 95)−54 (−702 to –36)−42 (−53 to –30)−8 (−22 to 6)
 20 mph local and main streets1757613585 (539 to 634)335 (300 to 373)−250 (−288 to –212)−43 (−48 to –38)−10(−19 to 0)
 30 mph934378311 (277 to 348)207 (180 to 237)−104 (−133 to –75)−33 (−41 to –26)REF
  • The rates (counts/total years) and difference in rates are accompanied by 95% CI using the Poisson distribution and the delta method.