AASHTO, FHWA, NACTO and other credible organizations recognize that 10’ general purpose lanes on streets in cities are fine. According to the best research by Eric Dumbaugh and others, 10’ lanes provide equal or superior results for safety compared to 11’ and 12’ lanes. Right-sizing the less safe 11’ and 12’ lanes to 10’ can also free up space for landscaped medians, bike facilities, sidewalks, on-street parking, or combinations thereof. During street design projects, the frustrating mystery is why so many transportation engineers still insist on wide lanes. Arguments fall on deaf ears about endangering motorists, walkers, and cyclists; promoting speeding; worsening business environments; increasing costs and impervious surfaces; and looking bad. Furthermore, engineers are supposed to be logical and science-based thinkers, making this situation even more baffling.
Here is my theory to explain the mystery. Before specializing in transportation engineering, many of us studied civil engineering. We were taught to design all sort of things, typically, with factors of safety so that our designs would not fail. Our beams would be a little thicker, our pipes a little bigger, our columns a little wider, and our foundations a little deeper than what was required for the loads. So, naturally we oversized our lanes too, you know, for safety. In the old days, 11’ and 12’ lanes were acceptable in cities due to ignorance. These days, however, we know better but it’s still hard for many engineers to believe that wide lanes actually build in a factor of danger, not safety. Lane widths don’t conform to the typical “bigger is safer” pattern.