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.

2 thoughts on “Chart of the Day: 4-3 Road Diets Safety vs. Congestion

  1. Walker AngellWalker Angell

    I would think that this would vary significantly based on the percent of traffic making left turns during the hour? Higher left turn rate = less impact of 4 to 3 diet?

    At what point will some motor traffic begin taking an alternate route? Instead of using a residential or retail area with high people-not-in-cars volumes as a throughway they use a highway as a throughway? Induced mand.

    What would happen if you replace this junction with a roundabout where there is enough space?

  2. Adam FroehligAdam Froehlig

    FHWA literature suggests that motor traffic will begin looking for an alternate route at around 20K vpd. What vph level that corresponds to is highly variable, but based on that 2nd chart I’d suggest somewhere in the vicinity of 1500 vph…but that’s going to depend a lot on two big factors: frequency of driveways and side streets, and signal timing, in particular the total cycle length and how much green time the 3-lane road gets in relation to that cycle length.

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