Comparing Crash Rates of Roadway Configurations

Every year, MnDOT publishes a number of safety statistics. A lot of that data is available here. One of the products they publish annually are the “Green Sheets,” which they publish for both various intersection types and roadway section types. The chart below is adapted from the 2013 Section Green Sheet. It shows the statewide crash rates and severity rates associated with various roadway section types.

Source: MnDOT

Source: MnDOT

Crash rate is expressed in units “crashes per million vehicle miles” on a particular roadway segment. Severity rates are unitless. It’s calculated the same way as crash rates, but crashes of greater severity are given a greater weighting in the formula. In other words, high numbers in this chart are bad.

MPLS-46th-St

46th Street, Minneapolis

This data has some limitations. The simple reporting measure strips away some important information about traffic volumes, access density, surrounding land uses, parking, roadway width, etc. But I think it’s still pretty illuminating by showing, on average, what type of urban roadways have higher crash rates than other roadway types.

MG-Weaver-Lake-Rd

Weaver Lake Road, Maple Grove

For me, the take home message in this data is just how problematic four-lane undivided roadways are. By a large margin, these types of roadways have higher average crash rates than any other configuration. It is also striking that the 3-lane roadways exhibit relatively low crash rates – substantially lower than any type of four or five lane roadway – lower even than two-lane roadways in some cases. In this data set, 4-lane undivided roadways exhibit crash rates and severity rates of 81% and 76% higher than 3-lane roadways, when junction-related crashes are included.

Burns Ave, Saint Paul. (formerly a 4-lane undivided)

Burns Ave, Saint Paul. (formerly a 4-lane undivided)

I have deep concerns about four-lane undivided roadways. They have had their time in the history of roadway design, but their time is up. We should work to eliminate them from our cities either by restriping as a 2-lane or 3-lane, or expanding to a 4-lane divided. Obviously, one of these options is cheaper and easier than the other, but there is a place for both.

My home isn’t far from a 4-lane undivided roadway (46th Street in south Minneapolis near I-35W), and it is easily my least favorite part of the neighborhood. It is directly adjacent to a grades 5-8 school, and it is terrifying to walk along or across with my children. I’m told Hennepin County may be looking at a mill & overlay on 46th Street sometime in the near future, and I’m hoping County staff will be amenable to restriping as a 3-lane. I will gladly endure any congestion that might result (though I think it is unlikely), in return for the improved safety conditions.

What conclusions should we draw from the data provided on the MnDOT Green Sheets? What are the limitations? Share your ideas in the comments below.

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11 Responses to Comparing Crash Rates of Roadway Configurations

  1. Sean Hayford Oleary
    Sean Hayford Oleary February 24, 2015 at 10:12 am #

    One problem somewhat unique to Minneapolis is that it has a fair number of undivided four-lanes that are de facto undivided two lanes most of the day. Cedar Ave north of 38th is one, 31st west of Chicago is another, and 46th between Nicollet and Chicago is as well.

    This design tries to do all things, but doesn’t do any particularly well. Cedar seems to see quite a lot of parked cars during off-peak, while 46th sees almost none (except at Bloomington and approaching Chicago Ave).

    Still, going to three lane means giving up parking 100% of the time — or if it’s a wider 4-lane like that Maple Grove example, it means choosing between bike space and parked cars.

    I’m not sure the answer is the same everywhere. On Cedar, think the impact of losing on-street parking would be really detrimental, at least north of 43rd or so. On 46th St, maybe not, but I could see businesses at Bloomington being concerned.

    Also, any idea why the urban two-lane has the highest crash rate that the lowest volume? That seems counterintuitive.

    • Sean Hayford Oleary
      Sean Hayford Oleary February 24, 2015 at 10:13 am #

      *at the lowest volume?

      • Reuben Collins
        Reuben Collins February 24, 2015 at 10:25 am #

        No, I don’t know what’s going on there. I’d be curious to find out.

      • Reilly February 24, 2015 at 11:18 am #

        Does the classification include narrower side streets (such as, say, Aldrich just south of Franklin)? The effective width of these streets is often narrower due to parking, and they often feature more conflict points due to driveways.

    • Reuben Collins
      Reuben Collins February 24, 2015 at 10:24 am #

      You’re right that parking is a big deal. Actually, I’ve seen more 4-to-3 conversions scrapped over parking impacts than anything else. However, assuming the cross section does not need to be symmetrical, a 4-to-3 conversion should still be able to accommodate parking with bumpouts on one side of the street. This can go a long way towards garnering public support. It’s only half as many parking spaces, but it’s allowed 24 hours/day, rather than the existing conditions of peak hour restricted parking.

      • Sean Hayford Oleary
        Sean Hayford Oleary February 24, 2015 at 10:47 am #

        That’s a good point, and might be a good option for 46th. (Although I believe the Bike Master Plan designates a route on 46th, so my preference would be to have bike lanes and parking only at business nodes.)

        That said, I know we looked at more creative ways to restripe on Nicollet Ave this year in Richfield, and Hennepin County was not interested in any creativity for a short, cheap project like a M&O. Portland was done with 3x 12′ lanes a few years ago, but Nicollet has less traffic and substantially fewer driveways, so dedicating 12′ of continuous space seemed like a waste when turn lanes were really only needed at intersections. The compromise was 11′ lanes instead of 12′, and the wide, 7.5′ shoulder allowing Sunday parking. Makes for a pretty nice shoulder for biking 6 days a week, but I can’t help but think we could have accommodated both bikes and parking better than we did.

  2. Reilly February 24, 2015 at 11:26 am #

    Just as a discussion point (no conclusion implied), is it possible that the higher crash rate on four-lane undivided roadways owes in some part to the greater proportion of higher-speed-limit roadways in that category (rather than to the lane configuration as such)? If 50 mph Eagle Creek Boulevard were dropped to 40, or if the existing 30 mph limit on Dale Street were enforced more visibly and/or strictly, how much would the disparity decrease?

    • Reuben Collins
      Reuben Collins February 24, 2015 at 11:41 am #

      Speed no doubt plays a role, though I don’t think there is a higher proportion of higher speed roadways in the 4-lane divided category than in the 4-lane divided or 5 lane category.

      Two data points MnDOT presents that I didn’t were crash rates for urban freeways and expressways (all higher speed roadways), which were 1.09 & 1.73 repsectively (including junction crashes).

  3. Bill Lindeke
    Bill Lindeke February 24, 2015 at 5:49 pm #

    this is exactly what i was saying! hey, do they have ped stats for this?

    it’s pretty basic, really. speed differentials X # of points of conflict = # crashes + severity. it doens’t take a rocket/road scientist to see that these streets are very dangerous. everyone who lives by one knows it in their gut.

  4. Mike Mackey February 25, 2015 at 9:40 pm #

    I think the data cited here is fairly useless when addressing a particular stretch of road. I live less than two blocks from 46th street and drive on it daily, and I honestly don’t see a problem.

    • Reuben Collins
      Reuben Collins March 13, 2015 at 2:45 pm #

      Hey, we’re neighbors! I drive on it nearly daily as well. Have you ever walked along it? I only do in a pinch, since we are lucky to have much more attractive places to walk. But the gas station at 4th Ave is the closest place to buy a gallon of milk to my home, so when my kids are screaming for milk, that’s where we walk.

      have you ever driven through there on a Sunday morning around 9:00 or so when all those folks are parking south of 46th but trying to cross to the church north of 46th? That gets a little dicey, too, don’t you think?

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