Wednesday, August 15, 2007

Livable streets v. passive safety

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.


Anonymous said...


I do not subscribe to a philosphy or a religion when it comes to public safety.
Please don't mischaractize my statements as being in the "passive safety vein".
If Mr. Aurbach is correct in his characterizing passive safety theory as "that driver error is random"; then I have to say you seem to be wasting quite a bit of time reading a lot of nonsense.
What I said was, a traffic sign would have sufficed to do what you intend, which is to slow traffic at the intersection of Daniel/ Jackson.
Instead you have the city spending lots of money to adjust the curb at this intersection, which ultimately will result in more accidents, hopefully small in nature, at worst, but quite possibly serious; as in head on, or jumping a curb when children are about.
It is my understanding this construction has already created problems, with the hitting of a gas line. Instead of all this fuss, a simple stop sign would stop the flow of traffic.
Your suggestion that stop signs do not do the job is just not cinsistent with my observations. I do not see drivers routinely ignoring and just blasting through stop signs throughout the Commonwealth, even when meeting your criteria. do people roll thru at times, not coming to a complete stop, sure, but the traffic is "calmed" as you say.

Adam said...

Neal, you should be aware that this is mitigation money from a developer that otherwise would not have been spent and be back in the developer's hands by now. Your characterizations of this project as unsafe are unfounded, and your comparisons with Commonwealth Avenue misplaced. You seem quite sure of yourself. Are you a traffic engineer?

The city hit a gas line because they started digging before digsafe finished marking the lines. This could have happened with any project, including the waterline they dug up on Daniel Street last week. Would you have blamed the homeowner? The consolation is that apparently the line they hit was the leaky one we had been smelling for years. Yes, we're lucky they didn't blow up the neighborhood in the process.

Laurence Aurbach said...

To track down more information specifically about the safety of traffic calming measures, the portal is a good place to start. Here is a brief summary of curb bulbouts.

Here are a couple of general resources about traffic calming measures:

Institute of Traffic Engineers, Traffic Calming: State of the Practice (1999)

U.S. FHWA, Pedsafe: Pedestrian Safety Guide and Countermeasure Selection System, September 2004. See especially Chapter 5.

And this study asks the question, "Do traffic calming measures improve safety?":

F Bunn et al., "Traffic calming for the prevention of road traffic injuries: systematic review and meta-analysis." Injury Prevention 2003;9; p. 200-204

Of course, no one can say whether a curb bulbout will improve safety without understanding the present and projected context in which it will be implemented. The opinion of an engineer who specializes in pedestrian and bicycle safety would probably be helpful.

Anonymous said...

First of all, just to correct Adam, I never said anything about Comm Ave., so I'm not really sure what you are referring to.

Additionally, I have to say that just because somoeone else is willing to pay, is no reason to waste money. I am sure we could have found a worthwhile use for the money.

As to the Sean's mention of Aurbach stating that passive safety assumes that driver error is random. If the statement is accurate , then the theory is without any merit at all. Driver error and accidents are never random. there is always a reason, perhaps not identifiable, but there is nothing scientifically that could possibly suggest a randomness in cause to driver error. If that were true there would be no difference in error based on age or sex. Insurance companies know all about this.


Adam said...

You're right. Re: Commonwealth, I misread.

It's not a waste of money to people who live in fear of traffic on their street. And we firmly believe the redesign will enhance life for everyone in the neighborhood and make it safer for kids walking to school. Those werer explicit design goals. And no, nobody found a use for the mitigation money or even tried.