Table 2

Comparison of collision rates—implementation versus control zones with 95% CI (Poisson distribution and delta method)

ZoneRate pre (20 mph zones)Rate post
(20 mph zones)
Rate pre
(matched control)
Rate post
(matched control)
Rate: % diff20 mphRate: % diffControlDiff in diff (%)
Collisions
 1a156 (132 to 182)115 (95 to 138)24 (15 to 36)18 (11 to 28)−26 (−36 to –16)−25 (−51 to 1)−1 (−29 to –59)
 1b21 (13 to 32)19 (11 to 30)15 (8 to 25)13 (6 to 22)−10 (−41 to 21)−13(−49 to 23)3 (−44 to 51)
 3284 (252 to 319)189 (163 to 218)117 (97 to 140)96 (78 to 117)−34 (−40 to –26)−18 (−31 to –5)−16 (−30 to –1)
 463 (48 to 81)43 (31 to 58)71 (55 to 90)75 (59 to 94)−32 (−48 to –16)6 (−15 to 26)−38 (−63 to −11)
 589 (71 to 110)48 (35 to 64)254 (224 to 287)220 (191 to 251)−46 (−57 to –35)−13 (−23 to –4)−33 (−47 to −18)
 669 (54 to 87)39 (28 to 53)106 (87 to 128)97 (79 to 118)−44 (−58 to –28)−9 (−25 to 8)−35 (−58 to –12)
 Total897 (839 to 958)593 (546 to 643)587 (540 to 636)519 (475 to 656)−34 (−41 to 27)−12 (−29 to 5)−22 (−40 to –2)
Casualties
 1a170 (145 to 198)125 (104 to 149)26 (17 to 38)20 (12 to 31)−27 (−36 to –17)−23 (−48 to 2)−4 (−30 to 23)
 1b25 (16 to 37)22 (14 to 33)16 (9 to 26)15 (8 to 25)−12 (−40 to 16)−6 (−43 to 31)−6 (−52 to 41)
 3323 (289 to 360)222 (194 to 253)144 (121 to 169)112 (92 to 135)−31 (−38 to –24)−22 (−34 to –11)−9 (−22 to 4)
 470 (55 to 88)48 (35 to 64)88 (71 to 108)95 (77 to 116)−31 (−47 to –16)8 (−11 to 27)−39 (−63 to –15)
 5103 (84 to 1250)57 (43 to 74)303 (270 to 339)248 (218 to 281)−45 (−56 to –34)−18 (−26 to –10)−27 (−40 to –13)
 678 (62 to 97)47 (35 to 63)126 (105 to 150)120 (99 to 143)−40 (−55 to –25)−5 (−21 to 11)−35 (−57 to –13)
 Total1014 (953 to 1078)680 (630 to 733)703 (652 to 757)610 (563 to 660)−33 (−40 to –26)−13 (−23 to –4)−20 (−22 to –8)