I’ve always been troubled by the way the press reports TTI’s annual mobility report. The press generally hones in on one performance measure (aggregate delay) when TTI models a bunch of performance measures that seldom get any air time. Total hours of delay overemphasizes population at the expense of really looking at how much of the urban geography is actually congested most of the time. So Los Angeles is a high-population metro region, and if a whole lot of people experience relatively minor delays, the aggregate delay calculation can look bad when, in fact, the delay experienced by any given traveler can be pretty minor. It’s not that total delay is uninformative, it’s just that total delay is only one measure when we need multiple measures of system performance to figure out what is going on.
TomTom has released their list of the 20 most congested cities using data collected from their devices, and the maps are based on data they are collecting from their GPS devices. LA by these data is #2: Seattle is # 1. It’s helpful to measure congestion as a function of geographic or network coverage because that is the way drivers or transit riders actually experience it. It’s a performance measure of network saturation, albeit a rudimentary one.
So…with the caveats: We have to question the sampling validity of TomTom’s data–after all, we’re only using data on people who own TomTom’s devices. We also have to question their method of breaking up cities within the same region: Oakland is part of the Bay Area; Long Island is part of the New York metro; Alexandria, Fairfax, and DC are in the DC metro, etc. So counting those as separate cities doesn’t really make sense, and it affects the baseline of their percentage calculation.
But we should look at Portland and Austin in particular, as those are largely freestanding cities. Portland is ranked 19th, which is pretty low on the top 20, but it only has about 600,000 people and 20 percent of its roads are congested. Austin, Texas is 15th, with 25 percent of its roads congested at a population of a little under 800K. Now we need to think about this because Portland is routinely used as a model of development, and that includes density as a means of “fighting congestion.” This is internally inconsistent, and we can’t be doing this. If we are going to aim for density, we are also aiming for congestion.
It should interest us that four of top 20 cities are DC- area metros: Fairfax, Alexandria, Montgomery County and DC proper–are on the list, despite the region’s extensive provision of rail. Population and density increases–a change that planners are committed to–bring about congestion on all urban systems: streets, sidewalks, roads, etc.
My point? My point is that if the sustainable city has population density, it also probably has congestion once it gets good-sized. Most of the arguments for density in urban planning, other than the social ones, are economies of scale arguments: density means intensive use of shared space and services. That also means congestion once populations grow as space and service are usually capacity-constrained.
What’s the answer? We need to make peace with congestion–not the easiest answer to sit with.