Comparison of collision rates—implementation versus control zones with 95% CI (Poisson distribution and delta method)
| Zone | Rate pre (20 mph zones) | Rate post (20 mph zones) | Rate pre (matched control) | Rate post (matched control) | Rate: % diff20 mph | Rate: % diffControl | Diff in diff (%) |
| Collisions | |||||||
| 1a | 156 (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) |
| 1b | 21 (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) |
| 3 | 284 (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) |
| 4 | 63 (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) |
| 5 | 89 (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) |
| 6 | 69 (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) |
| Total | 897 (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 | |||||||
| 1a | 170 (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) |
| 1b | 25 (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) |
| 3 | 323 (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) |
| 4 | 70 (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) |
| 5 | 103 (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) |
| 6 | 78 (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) |
| Total | 1014 (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) |