April 23


Guest post from Bryant Ficek, PE, PTOE, Vice-President at Spack Consulting

Four-lane undivided roadways are the most unsafe road type out there today (at least as measured by MnDOT highway crash stats here).  Urban four-lane undivided facilities in particular have a higher crash rate and severity rate than two-, three-, and five-lane roads whether urban or rural.  Beyond that basic stat, we also know that a conversion from a four-lane undivided road to a three-lane facility (one through lane in each direction with a center, two-way left turn lane) provides a host of safety benefits without negatively impacting operations (see studies here and here).

As I drove around my hometown of Roseville, MN one day, I noticed there seemed to be a lot of four-lane undivided roadways.  This made me wonder if the City could be significantly improved by converting one or more of them to three-lane roads.

To that end, I completed a small exercise to determine if there were roads that could undergo a “road diet” and switch from the current four-lane undivided design to a three-lane design.  In about two hours, I managed to complete the following three steps that showed the possibilities.

  1. Found the existing number of lanes for roads in the City through the City of Roseville’s Comprehensive Plan. The Comp Plan, and its Transportation Section, has a great map that shows all the roads in the City and how many lanes they provide.  Most cities in Minnesota are required to have a Comp Plan which would provide something similar to the one I examined for Roseville.  Hopefully other states/cities also have a similar document and information.
  2. Evaluated the identified four-lane roads using Google maps (Bing or another on-line map service would also work nicely). Each four-lane facility identified by the Roseville map was reviewed to determine if it indeed still provides four lanes and whether it is undivided or divided.  I was happy to see that City had already completed a road diet on some of the roads (the baseline map is from 2008).
  3. Examined the existing average daily volumes from the MnDOT traffic flow maps for each undivided four-lane road. This is something available on-line for most roads in Minnesota and most other states/cities I’ve worked in also have volume data on-line.  These volumes were compared against the planning level capacity of a three-lane road, which is about 17,500 vehicles per day.  Volumes above that suggest a three-lane roadway would not be sufficient.

The result is a map that suggests where a road diet could occur.  As shown below, 23 different segments were identified as potential candidates.

Does this mean all 23 should be re-striped tomorrow? Absolutely not.  Other factors need to be reviewed in more detail, such as the number of access points, crash history, parking, intersection operations, and what happens to the road on each end of the segment.  However, in about two hours, I have now provided the City Engineer a great starting point for increasing safety in the City (and potentially adding bicycle lanes on top of it!).

Mike’s Take:  I followed up with Marc Culver, Roseville’s Public Works Director/City Engineer.  He really appreciated this, although he pointed out most of these corridors are Ramsey County roads and he needs to work with them to get these roads converted.  Also, as a consultant – this is a great value add if you’re trying to market to a city or county.  

4-3 Lane Conversions in Roseville MN

  • Hi Mike,
    Interesting thoughts on changing 4 lane undivided roads to 2 lane with a center turn lane. In North Carolina, where I am now, there are many undivided 5 lane roads with the center lane being for turns in either direction. The amount of traffic pretty much requires the 4 driving lanes so the idea of limiting them to 2 is probably not reasonable. But I hate the middle turning lane. Much too scary since it allows movement in both directions in the center lane while also requiring beginning, elderly or non-expert drivers to make too many judgment calls about moving quickly from stop or near-stop to lanes with fast moving traffic. On a different subject, Jeanine and I gratefully accept you invitation to dinner in early June. Best wishes, Mark

  • Mark,

    You’re right – the five lane undivided roads with the two way left turn lane do not have the same safety record the 3 lane sections have.

    See you in June!


  • {"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}
    Mike Spack

    My mission is to help traffic engineers, transportation planners, and other transportation professionals improve our world.

    Get these blog posts sent to your email! Sign up below.