Hi,
I found this report today that looks at a 30km/hr vs 40km/hr speed reduction and compares collision and injury rates. It makes a pretty strong case for 30km/hr over 40km/hr, especially in a winter city.
Effect of reducing the posted speed limit to30 km per hour on pedestrian motorvehicle collisions in Toronto, Canada - aquasi experimental, pre-post study https://bmcpublichealth.biomedcentral.com/track/pdf/10.1186/s12889-019-8139-5
*Results*:Speed limit reductions from 40 km/h to 30 km/h were associated
with a 28% decrease in the PMVC incidencerate in the City of Toronto (IRR = 0.72, 95% CI: 0.58–0.89). A non-significant 7% decrease in PMVC incidence rates wereobserved on comparator streets that remained at 40 km/h speed limits (IRR = 0.93, 95% CI: 0.70–1.25). Speed limitreduction also influenced injury severity, with a significant 67% decrease in major and fatal injuries in the postintervention period on streets with speed limit reductions (IRR = 0.33, 95% CI: 0.13–0.85) compared with a 31% notstatistically significant decrease in major and fatal injuries on comparator streets (IRR = 0.69, 95% CI: 0.37–1.31). Theinteraction term for group and pre-post comparisons was not statistically significant (p= 0.14) indicating that there wasno evidence to suggest a pre-post difference in IRRs between the intervention and comparator streets.
*Conclusions*:Declines in the rate of PMVC were observed on roads with posted speed limit reductions from 40 km/hto 30 km/h, although this effect was not statistically greater than reductions on comparator streets.
Of note, in the results section they also note that while there was no difference between seasonal collision/injury rates on the 30km/hr streets, there was a marked seasonal difference on the 40km/hr roads, with winters having a far higher collision/injury rate.
Mark Cohoe Executive Director Bike Winnipeg t: 204-894-6540 e: mark@bikewinnipeg.ca