Lane widths might seem like a boring, esoteric aspect of traffic engineering, but they really matter for creating safe, livable streets. Noted city planner and urban designer Jeff Speck recently published a convincing piece on CityLab against a transportation feature that nearly all of us confront on a daily basis: 12-foot travel lanes. Over the past few decades, 12-foot lanes have become the traffic engineering standard, especially in newer cities and suburbs.

Speck’s literature review suggests that 12-foot lanes don’t improve safety compared to 10-foot lanes; in fact, there is little change, if not a decrease in crashes, in going from 12-foot to 10-foot lanes. There is also little change in vehicle capacity. What is certain, however, is that 12-foot lanes encourage speeding: by applying the same standards to city streets that we do for highways, people are more likely to ignore posted speed limits and drive the speed at which they feel safe. This has disastrous effects for pedestrians: a person hit by a car traveling at 30 mph is seven to nine times more likely to be killed than one hit by a car traveling 20 mph. Moreover, 12-foot lanes add an extra 20 percent of crossing distance (and collision exposure) for pedestrians: for example, at a six-lane road, pedestrians must cross 72 feet of traffic instead of 60 feet.

Speck concludes that 10-foot lanes cause no more collisions than 12-foot lanes, and may in fact cause fewer. Most importantly, these accidents are likely at a slower speed, and therefore less severe. For these reasons, 10-foot lanes are safer than 12-foot lanes.

What would a world look like with 10-foot lanes? Speck argues that drivers would drive more cautiously, more space would be available for bicycle amenities (like cycle tracks), and pedestrian safety/walkability would improve by creating a buffer from speeding cars. These changes are not very complicated, either: in some cases, all it takes is some paint.

But if 12-foot lanes are so bad, then why are they still being built? The biggest culprit is institutional inertia. While Mr. Speck shows that 12-foot lanes don’t provide a tangible safety benefit, the tricky reality is that it’s hard to empirically prove they’re unsafe – there are so many variables that contribute to street safety that it’s difficult to isolate lane width, which is why the literature is thin. Absent a convincing, conclusive study, the pace of change is likely to be much slower: city by city, agency by agency. And there will be many obstacles along the way outside of the old guard of traffic engineers: fire departments and bus operators, for example, are often supporters of 12-foot lanes to make it easier to maneuver their vehicles.

The good news is that a shift away from 12-foot lanes is slowly occurring. Cities like San Francisco have taken the lead in designing streets with urban-scale lane widths (many streets have 11, 10, or even 9-foot lane widths). NACTO has also provided a new voice to accelerate change over the traditional slow-moving establishment groups like AASHTO or ITE. It will be interesting to see how the conversation over lane widths evolves as a new generation of traffic engineers and city planners rethinks the standards of 20th Century American highways.