Chart of the Day: 4-3 Road Diets Safety vs. Congestion

Here are two charts for you, both on the 4-3 road diet topic. The first is from a Federal Highway Administration (FHWA)-Michigan Division powerpoint overview, and the second is from a 2012 Michigan-DOT study. They have quite different things to say about 4-3 road diets, and suggest some of the reasons that the design change might be controversial.

This first one is about crash rates, taken from this powerpoint:



This second chart is from the 2012 MDOT report:


Here’s the lengthy description of what you’re looking at (vph = vehicles per hour):

The graph below (Figure 2) shows the approach delay for both 4- and 3-lane sections (existing conditions [blue line] and road diet [red line]) as the mainline volume is varied from 750 to 2000 vph for a signalized intersection at site 9 (northbound direction). In this instance, the approach delay is less then 20 seconds/vehicle for both types of road when the mainline volume is less than 1000 but performance begins to degrade more rapidly for the 3-lane section for mainline volumes of 1250 and 1500 and very quickly beyond that. Graphs similar to this one are shown in Appendix C for each of the sites analyzed.

There you go! These are some of the trade-offs.

Bill Lindeke

About Bill Lindeke

Pronouns: he/him

Bill Lindeke has writing blogging about sidewalks and cities since 2005, ever since he read Jane Jacobs. He is a lecturer in Urban Studies at the University of Minnesota Geography Department, the Cityscape columnist at Minnpost, and has written multiple books on local urban history. He was born in Minneapolis, but has spent most of his time in St Paul. Check out Twitter @BillLindeke or on Facebook.