Laurence Aurbach on his Ped Shed site provides an invaluable framework for understanding the safety benefits of traffic calming .
The post, entitled Connectivity Part 7: Crash Safety, is too rich with information and insight to summarize adequately. I encourage everyone to read it in full.
One piece of Aurbach's analysis is useful in terms of addressing an objection to the the proposed Daniel/Jackson Street intersection redesign: the concern that that the proposed bumpout creates a new danger for motorists.
Aurbach compares two approaches to traffic safety (search for "safety first"). On one hand are the passive safety advocates, on the other are the livable (or complete) streets advocates. (Aurbach draws heavily on the work of Eric Dumbaugh's Safe Streets, Livable Streets: A Positive Approach to Urban Roadside Design (2005)).
Here's Aurbach on passive safety:
Passive safety assumes that driver error is random and impossible to predict, removes human judgment from the equation, and treats safety in a similar manner as structural engineering.
Passive safety calls for reducing all physical conditions that could conceivably be involved in traffic crashes. It means that anything drivers might crash into, like street trees, benches, parked cars and intersections, should be cleared away and minimized as much as possible. Guidelines based on the passive safety philosophy make livable streets difficult or impossible to construct.
In their objections to the bumpout, Barry Bergman and Neal Fleisher have made comments in a passive-safety vein. Mr. Bergman has said that the bumpout at the bottom of a hill introduces a new risk. Mr. Fleisher has said that the tighter confines of a redesigned intersection will lead to more accidents. Update: Mr. Fleisher rejects the characterization. See his comment to this post.
Livable Streets advocates, those that promote roadway design that enhances the pedestrian experience, argue that the additional potential hazards of livable streets—like the proposed bumpout—encourage drivers to exercise more caution, which reduces speeds, which reduces accidents.
The key factor in crash risk is design speed. Design speed is the speed at which drivers feel comfortable traveling; it is an entirely different concept than posted speed limits, which drivers usually feel are safe to exceed. Thanks to context, slow speeds prevail on livable streets. Drivers drive more slowly because the context signals the type of activity, amount of activity, and potential hazards that can be expected. Drivers “read” the context of livable streets and are impelled to exercise more caution.
Conversely, idiot-proof roadsides foster the illusion of safety and encourage speeding and lack of attention. High speed plus a lack of caution increases crash risk.
If you design to avoid or eliminate the risks that concern Messrs. Bergman and Fleisher, you make it comfortable for motorists to speed. And, they will.
Slower speeds are safer speeds. And, it's not just theory:
Traffic crash reports from a variety of countries are furnishing evidence that more pedestrian-oriented intersections, cities and regions are safer.
While I have a personal interest in the Daniel/Jackson intersection problem, the point here isn't just to promote my favored design. (Actually, it's my second favorite, but the City scotched the mini-roundabout.) I think that there is a larger point.
We need to recognize that the goal of traffic calming is to trigger the driver's immediate self-interest* to slow down. Stop signs don't trigger that immediate self-interest unless the stop sign is on an intersection where the motorist, by not stopping, risks hitting another car if he doesn't stop. Lowered speed limits don't trigger immediate self-interest because drivers respond to design cues in determining what's a safe speed, especially on roads they are familiar with.
We need to redesign our problem roadways to make drivers unwilling to drive fast.
*I say immediate self-interest because the threat of a ticket is too low to be a factor in driver attitude. The police couldn't do enough enforcement to permanently change driver behavior. There are frequent speed traps on Parker Street, for instance. Yet, Parker Street rarely moves at or below the speed limit.