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.
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