Access versus Mobility

I’ve been harping on the theme of access vs. mobility lately.

There’s a nice blog post out of Houston that puts this idea in stark relief, using London as an example.

Part of the equation is that if you have good access via proximity, the speed of your mobility is less important. That’s why the fact that our Streetcar is slow is not a critical issue (not that we shouldn’t keep working on making it faster).

If we have limited public dollars to invest in infrastructure, this begs the question: can we get more bang for our buck by investing in access (denser, mixed use neighborhoods and lower-cost mobility like Streetcars) or in faster, more expensive mobility (e.g., a new lane on I-205)?

In a recent interview in the Daily Journal of Commerce on the topic of place-making, Metro President David Bragdon is quoted on how expensive it will be to provide transportation infrastructure to new communities on the edge of the region like Bethany and Damascus. How does that contrast with investments to make our regional and town centers denser (i.e., provide more access)?

3 responses to “Access versus Mobility”

  1. Access, as used by Chris, would appear to be the product of average density over the area reachable within a specified travel time, times the area, where the relevant density is jobs, housing, retail activity, or whatever.

    The reachable area is directly proportional to speed for a linear corridor, and directly proportional to speed squared for a two-dimensional network.

    These relationships would apply to any travel mode.

    If a transit network is one-dimensional (no transfers) then it is true that increasing density has as much effect as increasing speed.

    However, a two-dimensional transit system may be significanly improved by increasing average speeds (with transfer times accounted for), which will increase accessibility proportional to the square of the factor of increase.

    To apply these relationships rationally to questions of public policy, one would need additional assumptions about the cost and desirability of both increased speed and increased density, as well as the social value of access. Perhaps the proponents of additional slow streetcar lines could explain their assumptions further?

  2. I don’t think the speed squared relationship is entirely accurate, as trips are usually constrained to a grid, which does not allow as-the-crow-flies travel.

    Let me try a different forumulation:

    access = density/speed

    You can achieve access with low density (sprawl) and high-speed roads, or you can achieve access with high density and lower speeds.

    Since energy consumption is a function of distance (not speed directly), the latter approach is inherently more energy efficient.

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