The traffic calming effect of narrower lanes is well documented.  Vehicle speeds reduce and safety increases as lanes narrow down to about 10.5 feet from the North American standard of 12 feet.

What would this narrowing mean in terms of cost for a typical new urban development?  About 10 percent of surface area for these developments is traffic lanes.  This excludes curbside parking, driveways, sidewalks, boulevards, parking lots, etc.  So, narrowing by 10 percent from 12 feet to 10.8 feet would free up 1 percent of land area. 

This is nice but we can do a lot better. If we were to replace most suburban two-lane streets with either one way streets or lanes with swales, the savings would be over 3 percent. And if we could replace arterial roads with through roads the savings would be another 2 percent.

 So now, we have reduced traffic lanes 50 percent, which means just 5 percent of a typical new urban development is devoted to traffic. With reduced speeds and road, surface mobility is bound to suffer. This is an enormous cost in time and money.

There are two ways that more than make up for the reduced mobility from narrow lanes.

First, beef up through roads (orange in the diagram).  Increasing the number of through road lanes, and increasing even flow with roundabouts, can increase average speeds and reduce peak speeds at the same time. 

Second, create a ride share transit network (purple in the diagram).  This creates even flow cross-town mobility at about 40 miles per hour with no intersections.

The cost of through road and rideshare networks is much less than the saving in street widths, drastically reduced signaling, reduced travel time, and most significantly, reduced injuries and property damage.

Compelling Evidence That Wider Lanes Make City Streets More Dangerous by Angie Schmitt Wednesday, May 27, 2015

“The rate of side impact crashes is lowest on urban streets with lanes about 10.5 feet wide — much narrower than the standard 12 feet.”  Graph: Dewan Karim. The “forgiving highway” approach to traffic engineering holds that wider is safer when it comes to street design. After decades of adherence to these standards, American cities are now crisscrossed by streets with 12-foot wide lanes. As Walkable City author Jeff Speck argued in CityLab last year, this is terrible for public safety and the pedestrian environment. A new study reinforces the argument that cities need to reconsider lane widths and redesign streets accordingly.  

White papers: Narrower Lanes, Safer Streets (Accepted Paper for CITE Conference Regina, June 2015)