Tuesday, June 8, 2010

More on Intersection Density

If for no other reason than to save it for later use, I came across a presentation by Norman Garrick who was probably the first that I'm aware of to start looking at empirical evidence based on intersection density, with the following table:

The numbers along the top are density of intersections per square mile area. As you can see, once you surpass 225 driving as modal share of transportation drops significantly. Not shown here is the drop in severe or fatal instances of automobile crashes over 81 intersections per square mile.

As I've discussed adding other levels of hierarchy to the study, Garrick includes distinguishing between minor intersections and major ones which he calls "nodes." He shows a direct relationship in increased safety to increased ratio of nodes to overall intersections, meaning that there is less hierarchy, not few major streets and many little streets, but a better balance.

Interesting. I shall keep that in mind moving forward as I mentioned this exact deficiency in my study worrying that two minor streets might create a four-way intersection and be overvalued as compared to two arterials intersecting with regard to convergence. Duly noted.