December 18, 2006
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)?
December 18, 2006 6:20 PM
Doug Allen Says:
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?
December 18, 2006 6:36 PM
Chris Smith Says:
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.
December 18, 2006 11:23 PM
"lower-cost mobility like Streetcars"
Of course it could be said it's cheaper than a BMW, but in reality it isn't much cheaper than a BMW w/ 3 people in it on average. But seriously, The average car price is 22-24k w/ an average mileage of about 20-22mpg. That energy consumption with an average of 1.2 people per car is better than many optomistic estimates on the Streetcar.
Saying it is a "lower cost" option is a very bad argument to make public. It would be better to argue on the simple idea that it makes denser development "possible".
...ugh. The endless misconceptions.
December 19, 2006 6:30 AM
Chris Smith Says:
I think I can reasonably argue that if you consider the total lifecycle costs to deliver a given amount of 'access' (including both the public costs and private costs like buying your car, insuring and fueling it) that Streetcar is cheaper than either LRT or freeway lane miles.
Part of that equation is that Streetcar helps create an environment in which (largely fueled by private sector investment) many more land uses are available to access in a much shorter travel distance.
To put it another way, over the next 20 years we need to build the public infrastructure to support 1 million new residents in the region. The public infrastructure costs (indeed the total public and private costs) will be lower if we do that in dense neighborhoods connected with Streetcars than if we do it in suburban subdivisions connected by freeways.