The Canadian Institute of Transportation Engineers published a study that found that crash rates and, more notably, side impact and turn-related crashes are lowest where the average lane widths at intersection approaches were between 10 and 10.5 feet. By analyzing vehicle-to-vehicle crashes at 70 signalized intersections in Toronto and 190 in Tokyo over four to five years, it found that intersections with lanes either narrower than 10 feet or wider than 10.5 feet had the highest crash rates.

Presented at the group's June 2015 annual meeting of the Canadian Institute of Transportation annual meeting, these findings are supported by other recent studies but challenge the commonly-held assumption that wider lanes are safer.  Some key findings include:

• Narrower lanes have higher capacity than higher, wider lanes and that lane width has no difference on the capacity for carrying safety and large-vehicles. 
• Pedestrian volumes decline as lanes widen, and intersections with narrower lanes provide the highest capacity for bicycles.       
• Human behavior is impacted by the street environment, and narrower lanes in urban areas result in less aggressive driving.

The authors suggest that designers of streets utilize the "unused space" to provide an enhanced public realm, including adding cycling facilities and wider sidewalks, or to save money and decrease the amount of impermeable surface through the use of less asphalt.         

And although narrower, 10 to 11 feet lane widths are sanctioned in national policies outlined by AASHTO, official standards in many U.S. states prohibit them.  According to a 2010 study published in the ITE Journal, six states require a minimum of 12-foot lanes and another 24 states require 11-foot lanes.