Older streets are safer

1874-platSomeone asked me recently to explain why the Planning Commission “doesn’t like culs-de-sacs”. I have a fondness for culs-de-sac and used to live on one, but they do have some serious shortcomings which, to me, outweigh their pleasant aspect. I was interested to read some new research that clearly quantifies ways in which our American suburban street model, which so dominated the second half of the 20th century, is in fact more dangerous than the traditional grid. (continued)

The newer cities tend to have more “dendritic” networks — branching, tree-like organizations that include many cul-de-sacs, limiting the movement of traffic through residential areas. They also don’t have as many intersections. The pre-1950 cities, on the other hand, tend to be more grid-like, giving motorists many more routes to choose from.

The ASCE study also concluded that street networks containing many cul-de-sacs increased travel demand on arterial roads by 75 percent and on collector roads by 80 percent, compared to a gridded street design. That, too, may help explain the higher fatality rate associated with the street networks that became prevalent after 1950.

You can read the original article here, then come back to comment.

As background, the Project for Public Spaces website has an excellent overview of some of the changes that are happening in the field of transportation planning (they call it a “revolution”), which shows how we got here and what we can do to reverse some of the damage.

What do you think? Are the arguments compelling, or not?

9 thoughts on “Older streets are safer”

  1. “New Urbanism” researchers often frame street design in terms of “grid” versus “culs-de-sac.” It is a fruitful opposition, because it makes it easy to visualize overlapping layers of “connectivity”–motorized transport, public transit, pedestrian movement, social connectivity, to name a few, within neighborhoods. Streets consist, in this view, of segments and nodes; segments being lengths of street and nodes the places where streets cross. Traditional grid-pattern streets have a higher ratio of nodes to segments and therefore have greater connectivity. The cul-de-sac warrens of suburban street planning have far fewer nodes and less connectivity.

    It is important to be clear about what sort of “connectivity” one is talking about: is it more routes for cars to reach a destination? Or better walkability or bikeability ? Or better connections between trails and parks? Or better opportunities for neighbors to connect with other neighbors? Or some of each? Ultimately, we would wish for a street system that somewhat deemphasized motorized traffic in order to boost walking and biking, help the environment, and promote social networking.

    What type of street system, grid or cul-de-sac is safer for children, pedestrians, bicyclists, in what is still a motor-driving world? The study by Marshall and Garrick reviewed in the article cited by Tracy Davis suggests that a grid system is actually safer because density of intersections is higher and cars must therefore travel more slowly. (This is not an ASCE study, by the way.) The argument seems inapt, because it is based on car crash data, not on pedestrian fatalities, and the principal cause seems to be speed, not density of intersections. In fact, intersections could probably be replaced by speed bumps to reach lower crash rates. The Marshall and Garrick study does not appear to show data on crashes in a hierarchical (cul-de-sac system), and would not indicate where in such a system the crashes took place. I conjecture that they happen on collector streets and arterials where cars travel at much higher speeds. So, it is quite possible that at the dendrite end of a local cul-de-sac, safety is quite good.

    Suburban cul-de-sac streets have long since fallen from grace for a variety of reasons—too few nodes with autos piling up at collector points, low walkability, isolation of homes, poor land use, among them. But researchers have generally found streets that end with culs-de-sac safer. A study by Eran Ben-Joseph, “Livability and Safety of Suburban Street Patterns: A Comparative Study” (Institute for Transportation Studies, UC Berkeley, 1995) found that cul-de-sac streets in Berkeley neighborhoods had 73% fewer accidents than grid streets (18 versus 68 accidents). The poor cul-de-sac may have gotten a bum rap!

    See also “Reconsidering the Cul-de-sac” (2004) by Michael Southworth and Eran Ben-Joseph (web.mit.edu/ebj/www/access24.pdf). The authors here assert that “The cul-de-sac model has several advantages that are worth considering. From the perspective of residents, the pattern usually offers quiet, safe streets where children can play with little fear of fast-moving traffic. A discontinuous short-street system, unlike the grid, may promote familiarity and neighboring.”

    On the whole, it would seem that grid neighborhoods and cul-de-sac communities both have advantages and drawbacks. Street planners, clever people that they are, have come up with the “Fused Grid,” which captures the advantages of each by inserting green-space parklets with walking/biking paths into the lock-step grid (see http://www.cmhc-schl.gc.ca/en/inpr/su/sucopl/fugr/index.cfm). Come to think of it, closing the road at Way Park creates just such a fused grid!

  2. As a resident who lives on a cul-de-sac street I can tell you with two young kids I would not want to live anywhere else. It is fun to be able to bike up and down the block with and go around the cul-de-sac umpteen times in a row without fear of cross traffic.

    I think that our neighborhood is enriched by the cul-de-sac as the neighborhood familes quite often gather in the street to chat and enjoy each others company.

    Northfield needs to think about building ‘neighborhoods’ as much as they need to think about traffic flow.

  3. Tracy –

    I remember that when I was on the Planning Commission we were told by city staff that it cost much more to do snow removal on culs-de-sacs as it does for the grid. Many of the Commissioners at the time came to the conclusion that perhaps the city couldn’t afford culs-de-sacs.

    • Ross
  4. I don’t think one group of people should be trying to link up everybody based on whatever anyway. Let things take their own course because eventually they will anyway. Just get the utility and safety issues dealt with and then live and let live.

  5. I grew up on a cul-de-sac and enjoyed being able to play in the street (mostly) without worring about traffic (except for when cars did come, they came fast. I believe because of the wide streets that often accompany cul-de-sacs when compared to older, side-walked neighborhoods). However, now that I have young kids and in terms of thinking about where to live in Northfield, I’d have a hard time giving up sidewalks because of how they allow my family to walk more safely around town. I understand that cul-de-sacs often have walking paths nearby, but they often require walking in the street to get there and they limit the places and directions that we can walk when compared to sidewalks.

  6. Good post, Tracy. You and David Sudermann give us a lot to think about, and I’m thankful for that. I quickly scanned the referenced articles. I don’t feel that I’m expert enough to fully judge the merits of the grid vs. fused grid system, but both move us away from where I think we’ve been as a city.

    I think a greater problem than cul de sacs is entire isolated neighborhoods such as Mayflower Hill east of the golf course and the neighborhoods off of Co. Rd. 1 south of town. These have no connections to Northfield except on roads that are safely accessible only by motor vehicles.

    “Accessibility” is also something that transportation researchers are examining and trying to measure. The U of M’s Center for Transportation Studies is working a lot on that topic.

    The study David points us to is Canadian, and I’ve found in some other research I’ve done that Canada is generally ahead of us in transportation policy and planning.

    Anyways, I’m glad that we’re “multi-modal” in our transportation thinking here in this post. As we set about stimulating our national economy with transportation projects, it’s important that our leaders think “multi-modal” too. I think they’re only part way there now, and the general emphasis is on highway spending.

    (Note that PPS stands for Project for Public Spaces, not Partnership for…)

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