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I highly recommend this article on the history of the 85th percentile speed limit "rule," by Brian Taylor and @Y___Hwang . Offering below my thoughts on its rural roots and also the conundrum (and travesty) of it getting applied to urban contexts. https://journals.sagepub.com/doi/10.1177/0361198120928995
In the 1920-1930s, even 1940-1950s in some states, rural roads were under constant improvement. Moreover, vehicle capabilities were also constantly changing. There is a logic, therefore, to "crowdsourcing" speed limits for open stretches of road. (pix: Library of Congress)
The mystery, which this article leaves for future research, is how in the world does the 85th percentile rule get accepted for urban streets and roads?!?!?! For whoever does this research, I offer that "crowdsourcing" is NOT the recommended approach in 1958.
The above chart is from Better Transportation for Your City by the National Committee on Urban Transportation. US municipal, state, and federal gov'ts are all involved. It recommends planning urban speeds according to street function and SAFETY. Link: https://hdl.handle.net/2027/wu.89046875142
Of significance: it recognizes three types of local streets-- business, residential, and industrial--that should have very low speeds. Even collector streets have low speeds. The text makes clear that SAFETY is the top priority on local and collector streets.
What is meant by "overall" speed is unclear, but it is NOT either our modern concept of design speed nor average travel time. It could be an observed speed, but the speed itself is prescribed.
The guide also integrates mass transit and parking recommendations. For pedestrians: they offer recommendations for safety islands and the like for busy, faster streets. And my favorite: sidewalks on BOTH sides of the streets for almost everywhere.
And the kicker: the name for this paradigm that addresses both vehicle speed and road-user safety? They call it level of service.
Whatever went wrong with planning America's urban streets happened after 1958.
Whatever went wrong with planning America's urban streets happened after 1958.
