pbot-20-mph-speed-study-finalv5.pdf (13.2 MB)
This paper provides detailed before and after traffic observations when Portland moved to 20mph speed limits. Appendix B shows how the highest speed tails of the speed distributions are squashed after the 20mph limit.
It is most noteworthy that the reduction in the percentage of vehicles faster than 30 mi/hr and 35 mi/hr are larger in magnitude than the other changes. These changes are more meaningful for the Vision Zero speed reduction efforts than the change in average speed, given the link to crash severity for vulnerable road users. For comparison, the 1% reduction in the proportion of speeds greater than 25 mi/hr was also observed by Hu and Cicchino (2020) in their study of 30 mi/hr to 25 mi/hr speed changes in Boston, MA. The reduction in the proportion of the speeds greater than 30 mi/hr is moderately greater in the present study.