Getting It Wrong in Montogomery County

As was recently reported in the Washington Post, Montgomery County, Md., is planning an overhaul of its “road code,” the sort of thing that seems like a bureaucratic footnote but then goes on to have major implications in the built environment.

Among the major issues, the newspaper reported:

The panel recommended that roads in urban areas be designed for speed limits of 30 to 40 mph, saying anything slower would be unrealistic and difficult for police to enforce. The panel also said trees should be planted farther from curbs on roads with 40 mph speed limits because of the danger they pose to motorists who hit them.

What strikes me in discussions like these is the weird disconnect between design and driver behavior. One of the reasons it can so often be difficult to enforce lower speed limits is that these limits are posted on roads that are intensely over-engineered. The supposed “fix,” as suggested above, is to assume that drivers are going to drive at a certain speed, and so to then rearrange the entire landscape — removing trees, etc. — to allow them to do so “safely.”

Of course, on the road “designed” for speed limits of 30 to 40 mph, they will inevitably drive faster. But then, of course, if someone crashes and kills a pedestrian or another driver, it’s an “accident,” it’s down to driver behavior; if they smash into a tree, it’s deemed poor traffic safety engineering. As the work of Eric Dumbaugh has found, looking at streets like the one above, at Stetson University in Florida, often the worst safety performance comes on the roads that are deemed “safe” by traffic engineers, while the best can come on tree-lined streets like the one above (which had no crashes and speeds below 30 mph during the five years he looked at it).

We consistently get urban speeds wrong in the U.S. In Germany, the land where speed is supposedly worshipped, the speed-limit free sections of the autobahn are contrasted by a mandatory, heavily enforced 30 KPH (that’s 18 mph, folks) limit in residential areas.

Another classic specter the article invokes is emergency response times. Any time a group seeks to lower speeds on a road, there are dark projections made of people being killed in fires because firefighters will be held up on traffic calmed streets. Well, for one, have you ever seen these vehicles on the way to an incident? They often don’t actually drive that much faster than anyone else — particularly since cars frequently don’t get out of the way in time — but I wonder if the lights and sirens and the panic they induce may make us overestimate their sense of urgency. In any case, studies have suggested that emergency-response teams are as likely to be help up by random traffic delays and the like as anything else.

But the larger issue is risk. As Reason magazine has pointed out, the risk of dying in a fire in the U.S. is roughly the same as drowning: In one year, 1 in 88,000, and, over a lifetime, 1 in 1100. The risk of dying in a car crash, according to the article, is 1 out of 6500 in a year. The risk of being killed while being a pedestrian? “A one-year risk of one in 48,500 and a lifetime risk of one in 625.”

Designing roads to meet some supposed emergency response criteria, for that dramatic last-second rescue, actually helps raise the risk of dying in a much more common way: In traffic.

This entry was posted on Tuesday, October 21st, 2008 at 8:41 am and is filed under Cities, Drivers, Pedestrians, Risk, Roads, Traffic Engineering, Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

9 Responses to “Getting It Wrong in Montogomery County”

  1. Vincent Clement Says:

    The way I look at it, I’d rather have vehicles hitting trees than hitting pedestrians or smashing into houses. Trees are a very attractive and effective safety barrier. I also hear that they cool surrounding area, clean the air and provide a habitat for other living creatures.

    Nothing drives me more crazy than designing for emergency response.

    Our fire department does not like hammerhead turnarounds – takes ‘too long’ for them to turn around. So instead we build huge cul-de-sacs that are the size of half a hockey rink. What a waste of space and resources.

  2. Gary B Says:

    I’m not sure if you’ve seen this before, but I thought I’d pass it on. An image listing the “Total odds of dying, any cause” was posted on the website in August ’06 and was available there until June ’07. The image is still available from through this link: can sometimes be a little flakey. If it’s acting up, I have a copy of the immage at the bottom of this blog post.

  3. Peter Says:

    closer to 19 mph, but i get the point:

    30 kph = 18.6411358 mph

    The only thing I’m trying to figure out is, is 20 MPH too high a limit to allow here in the US, or should we go ahead and advocate for 18 MPH or less?

  4. Peter Says:

    Good post – we should design streets for real conditions, not hypothetical situations that are only fun to talk about in class.

    the 2nd-to-last paragraph is ill-formed, and i’m not sure it would make sense even if it was syntax-correct.

  5. David Hembrow Says:

    You see just this contrast between British and Dutch roads. Britain always designs for cars to travel at speed, while the Dutch have lower speed limits combined with trees etc.

    One of the consequences of the UK putting driver’s convenience and speed above all else is that street furniture provided for drivers (e.g. street lights, signs, traffic lights) end up located in the middle of the sidewalk so that they’re not “dangerously” close to the road.

    This makes the sidewalks less attractive for pedestrians, and leads to more driving.

  6. Charles Komanoff Says:

    Excellent, important post, particularly the important rebuttal to the “emergency response time” canard. But you may have omitted the greatest cost of over-engineered, high-speed streets: the discouragement of physical activity (biking and walking) and the consequent missed opportunity to foster improved physical and mental health.

  7. Karl-On-Sea Says:

    . . . and of course, there’s some screwy thinking going on in most of our heads as soon as we get behind the wheel. A 30mph speed limit should be perfectly sensible. Except most people treat it as a LOWER rather than an UPPER limit.

    Echoing David’s point – here in Whitley Bay, there’s a parade of shops. The road’s too narrow for parking on both sides, so people used to park with two wheels up on the pavement [sidewalk]. To prevent this, there’s now a row of steel bollards – set about a foot from the edge of the road, taking up as much of the pavement as if there was a car parked there in the first place, but now as a permanent feature. Just sometimes, I wonder if the lunatics really are running the asylum!

  8. SI Units Says:

    “30 kph = 18.6411358 mph

    The only thing I’m trying to figure out is, is 20 MPH too high a limit to allow here in the US, or should we go ahead and advocate for 18 MPH or less?”

    You should advocate for SI units :)

  9. Andrew Curry Says:

    In response to the questions about 20 mph limits, most of the UK traffic research looks at SI units (20/30/40 mph) and the difference in pedestrian impact at 20mph is sufficiently improved (in terms of likelihood of surviving, not being badly injured etc) to not worry about shaving an extra couple of MPH off. (There are increasing numbers of residential streets with 20 mph limits, although they are under-enforced).

    On enforcement, I wonder if it needs a specialist and separate Traffic/Transport Police Division before it’s taken seriously as an issue? In the past when I’ve talked to people who’ve spent some time as traffic cops they say that it is usually the lowest status part of a UK police force. No wonder, then, that they don’t take it seriously, even though – as some of the discussion above points out – we all have a higher chance of dying in traffic than, say, by being knifed (which the police attach far greater priority to).

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