How did the protected bike lane suddenly become common in America? Advocates will tell you it was the result of stalwart activism. And trailblazing, politically daring transportation officials did play a part in bringing better bike lanes to the nation. But the spread of bike lanes to so many corners of the country couldn’t have happened without a simple, ordinary technology: a set of street-design standards, written down in a book so that less daring engineers didn’t have to blaze their own trails anew.

Street-design standards are a labor-saving and risk-reducing technology that memorialize the expert judgment of the traffic-engineering field. Designing a street includes an enormous number of judgment calls, having to do with questions of space, time, and information.

Geometric standards, like the width of traffic lanes and tightness of curves, come from the American Association of State Highway and Transportation Officials (AASHTO), which publishes the “Green Book” (aka A Policy on Geometric Design of Highways and Streets) and other road-design guides. Standards for signs and signals come from the Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD), which is published by the federal government. The MUTCD has the force of law, which is why traffic lights and road signs look virtually identical across the country. For example, an American stop sign must be a red octagon, no less than 30 inches square (36 inches on a multilane road). AASHTO’s guidebooks set standards on large, federally regulated roads, but only issue advisory guidance for urban streets. Still, they have enormous influence and are often treated as standards.

Street-design standards are essential to minimize confusion among people traveling from place to place. Only the largest cities have much in-house ability to design new street projects. A city of 130,000 people might have no more than a half-dozen engineers and planners who play any significant role in designing streets. Standards also help shield cities from legal liability. Generally, a city cannot be sued by a person who crashes into a tree and blames the road if that road was constructed in accordance with prevailing standards.

But standards can also replicate failure. Sometimes, that’s because codified “expert judgment” turns out to be wrong. But it can also be because standards embed value judgments in ways that are rarely examined. The traffic engineer Chuck Marohn has argued that the dominant street-design standards in America overvalue the speed and flow of automobile traffic, at the expense of pleasant neighborhoods that are walkable, bikeable, and financially productive.

For example, AASHTO books generally recommend up to 12-foot traffic lanes, which feel safe to drive at 40 miles per hour.* But in dense cities, where the speed limit may be 25 miles per hour or less, it would be better to make speeding feel uncomfortable for drivers by striping narrow, 10-foot lanes for cars. Similarly, the Northeastern University professor Peter Furth has pointed out that on roads with medians, the MUTCD doesn’t require pedestrian signals to provide enough time for people to fully cross the street—only enough time to make it to the center median.