Photo by the author.

DC resident Jeff Speck wrote Suburban Nation, the best-selling book about city planning since Jane Jacobs. Greater Greater Washington is pleased to present 3 weekly excerpts from his new book, Walkable City: How Downtown Can Save America, One Step at a Time.

Contrary to perceptions, the greatest threat to pedestrian safety is not crime, but the very real danger of automobiles moving quickly. Yet most traffic engineers, often in the name of safety, continually redesign city streets to support higher-speed driving.

This approach is so counterintuitive that it strains credulity: Engineers design streets for speeds well above the posted limit, so that speeding drivers will be safe—a practice that, of course, causes the very speeding it hopes to protect against.

Even my old South Beach neighborhood, known for its walkability, was not immune to this sort of thinking.

If you have seen the remake of La Cage Aux Folles, you might remember the lively streetscape of Espanola Way, where Robin Williams buys an elaborate birthday cake for his partner. Follow that street two blocks west, and you will find that already-narrow sidewalks have been cut in half in order to widen a roadway that functioned perfectly well before. Why? Because the standards had changed—from walkable to not.

I have never heard a proper explanation for the creeping expansion of America’s street standards. All I know is that it is very real, and that it has a profound impact on the work that city planners do every day.

In the late nineties, I was working on the design of Mount Laurel, a new town outside of Birmingham, Alabama, that was modeled on that city’s most successful prewar neighborhoods. We had measured the streets of Homewood, Mountain Brook, and the city’s other best addresses, and planned our thoroughfares with the same dimensions. We were then told that our streets did not meet the standard, and our engineering firm was unwilling to stamp the drawings for fear of legal liability.

I remember one particular afternoon, when we convinced the County Engineer to tour these great neighborhoods with us in our van. Perhaps anticipating our consternation, he gripped the door handle with white knuckles and shouted “We’re gonna die!” as we motored calmly around the narrow, leafy streets of Mountain Brook. I’m pretty sure he was joking, but his ultimate pronouncement was clear: we had to re-engineer our streets with a higher design speed.

This logic—that higher design speeds make for safer streets—coupled with the typical city engineer’s desire for unimpeded traffic—has caused many American cities to rebuild their streets with lanes that are 12, 13, and sometimes even 14 feet wide. Now, cars are only 6 feet wide—a Ford Excursion is 6’-6’‘—and most Main Streets were historically made of 10-foot lanes. That dimension persists on many of the best, such as ritzy Worth Avenue in Palm Beach, Florida. Yet, many cities I visit have their fair share of 13-footers, and that is where much of the speeding occurs.

For me writing this, and you reading it, it is undoubtedly clear that building wider lanes would cause drivers to speed. After all, if highways have 12-foot lanes, and we are comfortable negotiating them at seventy miles per hour, wouldn’t we feel the same way on a city street of the same dimension? Yet, in the bizarre parallel universe of the traffic engineer, no such relationship exists. Motorists will drive at the speed limit, or slightly above, no matter what sort of drag strip we lay in their path.

As with induced demand, the engineers have once again failed to comprehend that the way they design streets will have any impact on the way that people use them. By their logic, just as more lanes can’t cause more driving, high-speed lanes can’t cause high speeds. Ladies and gentlemen, allow me to introduce you to the second great misunderstanding that lies at the root of most urban degradation today. Widening a city’s streets in the name of safety is like distributing handguns to deter gun violence.

Just in case you think I am making this up, let’s turn to the calm analysis of Reid Ewing and Eric Dumbaugh, professors at the University of Maryland and Texas A & M, respectively. In their 2009 study, “The Built Environment and Traffic Safety: A Review of Empirical Evidence,” they assess the situation this way.

Considered broadly, the fundamental shortcoming of conventional traffic safety theory is that it fails to account for the moderating role of human behavior on crash incidence. Decisions to … widen specific roadways to make them more forgiving are based on the assumption that in so doing, human behavior will remain unchanged. And it is precisely this assumption—that human behavior can be treated as a constant, regardless of design—that accounts for the failure of conventional safety practice.

How costly is this failure? In another study, presented at the 80th Annual Meeting of the Transportation Research Board, Rutgers professor Robert Noland calculated that increased lane widths could be blamed for approximately 900 additional traffic fatalities per year.

We can only hope that these studies eventually have an impact on thoroughfare engineering as it is currently practiced in the typical American city. Currently, engineers still deny their stamp of approval to streets configured without “adequately” high design speeds. “We’re afraid of being sued,” they say.

Some day, I might get up the nerve to respond as follows: “Afraid? You should be. Now that we’ve publicly presented to you that narrower roads save lives, we are going to sue you when people die on your fat streets.”

There is some good news. Thanks to the labors of the Congress for New Urbanism, a nonprofit focused on making more livable cities, we have made a start in changing the standards. The CNU teamed up with the Institute of Traffic Engineers to create a new manual, “Designing Walkable Urban Thoroughfares,” that recommends street lanes of 10 and 11 feet wide. With the imprimatur of the ITE, this book can now be waved at planning meetings in support of more reasonable standards. I just wish that “11” wasn’t in there.

Another cause for hope is the growing “20’s Plenty for Us” movement that, having taken the United Kingdom by storm, is just beginning to win followers in the US. Recognizing that only 5 percent of pedestrian collisions at 20 miles per hour result in death, vs. 85 percent at 40 mph, the British have introduced 20 mph speed limits in many of their cities.

There are currently more than 87 “Twenty’s Plenty” campaigns in the UK, and about 25 British jurisdictions, with a combined population of over six million, have committed to a 20 mph speed limit in residential areas. In June, 2011, the European Union Transport Committee recommended such a rule for the entire continent. It is easy to imagine 20 mph becoming a standard throughout Europe in the near future.

On this side of the pond, Hoboken, New Jersey, may be the first city to have instituted a “Twenty is Plenty” campaign. Unfortunately, in true Jersey fashion, the 20 is just a suggestion, while higher official speed limits remain in place. As I write this, New York City is pioneering some legitimate 20 mph zones.

These developments are important—but not as an end in themselves. As any London pedestrian will tell you, a 20-mph sign does not a 20-mph driver make. Most motorists drive the speed at which they feel comfortable, which is the speed to which the road has been engineered. “20’s Plenty” is most useful as a first step to slower design speeds. Once 20-mph zones proliferate, we may finally be able to convince the engineers to design 20-mph streets.

Speck’s book comes out on November 13. You can pre-order it on Amazon. For more from the book, see also our first excerpt.

Jeff Speck is a city planner who, through writing, public service, and built work, advocates internationally for smart growth and sustainable design. As Director of Design at the National Endowment for the Arts from 2003 through 2007, Mr. Speck oversaw the Mayors’ Institute on City Design, and created the Governors’ Institute on Community Design. Mr. Speck spent the prior 10 years as Director of Town Planning at Duany Plater-Zyberk and Co., and currently leads a boutique design consultancy based in Boston.