Lane Width

The width of a travel lane on a roadway greatly influences the safety and speed of driving, as well as the total width of a roadway. The roadway width in turn determines the pedestrian crossing distance and the roadway width potentially available for other uses such as bike lanes, parking lanes, or landscaped curb extensions. Conventional traffic engineering theory holds that 12 feet is the optimal lane width, whether for highways, urban streets or rural roads. For example, the Missouri Department of Transportation states,

Narrow lanes force drivers to operate their vehicles laterally closer to each other than they would normally desire...In a capacity sense the effective width of travelway is reduced... In addition to the capacity effect, the resultant erratic operation has an undesirable effect on driver comfort and crash rates.


Bigger Is Not Better

raised_intersection-burden.jpgDespite the accepted engineering wisdom, narrower lane widths are actually associated with fewer traffic injuries and fatalities. A large study of crash data from all 50 U.S. states over 14 years revealed the following:

* Those states with more arterial roads with lane widths of 9 feet or less had fewer traffic injuries. Lane widths of 10 feet or 11 feet are also associated with fewer injuries and fatalities compared to 12 foot lanes.
* For collector streets (residential streets that provide vehicle access to large arterial roads), the same pattern is found. That is, 12 foot wide lanes are associated with more injuries and fatalities.
The author concludes, "these results are quite surprising, as it is general practice to improve the safety of roads by increasing lane widths. One possible behavioral response is that drivers increase their speed when lanes are wider and offset any safety benefit from increased lateral spacing. Another possibility is...that drivers may feel safer and reduce cautionary behavior."

In another study, a researcher found that two segments of the same road, that differed primarily in lane width and adjacent land uses, had 31% fewer crashes during the five years of 1999-2003. The lane width of the narrower section was 11 feet, while it was 12.5 feet in the wider section. Traffic was faster on the wider section, which undoubtedly added to the crash risk.


Narrow Urban Lanes = Less Congestion

Street design expert Dan Burden points out that, at busy urban intersections, narrower lanes allow crossing pedestrians to clear the intersection significantly faster. This allows the green time for cars to be increased, reducing congestion.