Chart of the Day: Simulated Average Speeds Before and After 4-3 Conversions

Here’s a chart from a traffic engineering study (out of the University of Nevada*) that simulates the effects of 4-3 road diets on automobile speeds at stop sign controlled intersections.


The upshot of all this is that there are a lot of variables which enter into the picture when studying traffic impacts of 4-lane (Death Road™) vs. three-lane designs. For example: how many buses are there? What percent of the traffic is trying to turn left at two-way stop sign controlled intersections?

(The one thing that doesn’t enter into these LOS simulations, however, is pedestrian and bicycle traffic and/or safety concerns.)

There are a lot of different studies and arguments about the feasibility of 4-3 road diets, and at what point the tradeoff between safety and traffic speeds starts becoming one-sided. This kind of analysis is just one small piece of the puzzle.


*Note: I have no idea how peer-reviewed or thorough this study might be. It seems to have been presented at a convention-type thing in 2011. is a non-profit and is volunteer run. We rely on your support to keep the servers running. If you value what you read, please consider becoming a member.

2 Responses to Chart of the Day: Simulated Average Speeds Before and After 4-3 Conversions

  1. Alex Cecchini
    Alex Cecchini November 6, 2014 at 3:24 pm #

    How much of the average speed reductions came from design changes (eg narrowed lanes) vs a capacity problem? It seems like the gap between 4 lane/3 lane average speeds stays relatively constant across vehicle volumes. This tells me that capacity of a 3 lane to handle turning movements isn’t the problem as vehicle counts grow, but we should view the average speed reduction as a positive for safety reasons.

    Also, I’ve asked this before, but why do we view buses stopping in a traffic lane as such a negative? We have stop signs and lights that delay drivers so other divers can access/cross the street. Why should a vehicle stopping every (other! 🙂 ) block to pick up passengers be any different?

    • Walker Angell
      Walker Angell November 8, 2014 at 8:40 am #

      From a purely technical standpoint I’d think that each bus stop could cause more delay than each sign/signal stop. And thus you’re adding 50% to 100% more stops. If buses were completely pre-pay then I think that’d take care of a lot of this issue.

      The bigger issue is mental. Not a lot different than drivers being more frustrated and angry when delayed an overall 10 seconds having to drive slowly behind a bicycle rider than 50 seconds at a signal.

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