What Exactly is a Road Diet Anyway?

Image by Jeff Speck and Spencer Boomhower

A road diet, less commonly referred to as a lane reduction or lane rechannelization, is a technique in transportation planning designed to reduce the number of vehicle travel lanes on a roadway in order to achieve a number of benefits, such as increased safety or the provision of space for other modes of travel.

While “road diet” is a bit of a buzzword amongst walking/ cycling enthusiasts and transportation advocates, not everyone may be entirely clear on the different types of road diets and how they work. Enter city planner, author, and urban designer Jeff Speck, who collaborated with 3D artist Spencer Boomhower to produce four beautifully clear explanations of the four most common types of road diets.

Three lanes to two on a one-way street

Speck starts off with one-way streets that have 3 traffic lanes and 2 parking lanes – a serious amount of road space dedicated only to cars. He demonstrates how easily a cycle track can be added simply by removing one lane of vehicle traffic and pulling one parking lane 10 feet away from the curb, leaving space for a 2-way cycle track protected by the parked cars. Speck notes that a road diet of this sort on Prospect Park West in Brooklyn, NY resulted in a 77% reduction in speeding and a 63% reduction in injury crashes while having no impact on car volumes or travel times.

Four lanes to three on a 2-way street

Speck refers to this second technique as the “classic American road diet,” reducing a four way street to three lanes, with the center reserved for left turns in both directions. This method frees up 10+ feet up space which could be used for the addition of bike lanes on either side of the street, or sidewalk extensions. Speck notes that a study of 17 different 4-3 road diets in the US resulted in significant safety improvements with zero net decrease in vehicle capacity.

Bike lanes to protected bike lanes

While bike lanes are undeniably a great thing for cities, many bike advocates believe protected cycle tracks – not just regular bike lanes – are necessary for any meaningful attempt at increasing cycling’s modal share. In this technique, Speck demonstrates how painted bike lanes on either side of the street can be converted into a 2-way protected cycle track by moving them both to one side of the street and pulling the parking out 10 feet to act as a barrier. By reducing vehicle lanes from 12 feet to 10 (which Speck notes is safer and does not reduce capacity) this conversion is possible without losing any vehicle traffic lanes.

40-footer lane insertion

This last technique is as much about the addition of the bike lane as it is about slowing vehicular traffic on urban streets. Speck notes a recent trend in North American transportation planning is the design of 12 foot vehicle lanes, rather than the old standard of 10 feet. While it was thought that wider lanes would move traffic through cities more quickly, the extra 2 feet in fact just encourages drivers to speed. Speck presents the simple solution of adding a bike lane to one side of the road to produce a traffic-calming effect on city streets.