U.S. national average accessibility to jobs over time (1990-2010). Note that only a small % of commutes are longer than 60 minutes. Source: Center for Transportation Studies, Uni of Minnesota

Measures of traffic congestion in US cities typically focus on travel delay. Big dense cities like New York, San Francisco and Los Angeles invariably score poorly.

A new study by the University of Minnesota’s Center for Transportation Studies looks instead at accessibility, which measures “the ease of reaching desired destinations”.

Accessibility recognises that how close origins and destinations are to each other is at least as important for journey times as the speed of travel between them.

Lead researcher, Professor David Levinson, says:

There are two ways for cities to improve accessibility—by making transportation faster and more direct or increasing the density of activities, such as locating jobs closer together and closer to workers

Professor Levinson counted the number of jobs that could be reached within a range of driving times by the average worker resident in the 51 largest US metropolitan areas (see first exhibit).

He established an overall ranking of the 51 metro areas based on giving closer jobs a higher weight than more distant jobs. The most accessible metropolitan areas in order are (see second exhibit):

Los Angeles, San Francisco, New York, Chicago, Minneapolis, San Jose, Washington, Boston, Dallas, and Houston.

The average Angeleno can only reach 4.9% of metro jobs within 10 minutes driving time, but that very quick trip nevertheless provides access to a massive 237,203 jobs. This is only bettered by San Jose (237,929) and San Francisco (283,549).

In contrast, the average commuter in New Orleans can reach 32.8% of metropolitan jobs within a 10 minute drive. However that provides access to considerably fewer jobs (146,521).

With a longer 30 minute commute, the average Angeleno can drive to a mind-boggling 2,458,111 jobs. That’s more than five times the number (446,087) that the average New Orleanean can get to within the same driving time.

Although Los Angeles has high levels of traffic congestion (the TTI ranks it second behind Washington DC), workers can get to a large number of jobs within a reasonable time frame because both residential and employment densities are relatively high.

Indeed, Los Angeles has the second highest average population density of any metropolitan area in the US. And it’s also the second densest when measured by weighted density (New York is the densest US metro on both metrics).

While Los Angeles doesn’t have the high peak density of the centres of metros like New York and San Francisco, it doesn’t have as many residents living in very low density suburbs either. Activities are “spread out” at relatively high densities and connected by freeways.

Professor Levinson’s study also found that while the average American living in the top 51 cities could reach fewer jobs in 2011 than in 1990 in the same time, he or she could reach more today than in 2000 (see first exhibit). Average commute speeds were also faster in 2011 than in 2000 and about the same as they were in 1990.

Perhaps the most important takeaway from the study is that high levels of accessibility to jobs by car can be achieved in both dense and dispersed urban forms.

Six of the top ten most accessible cities are also among the top ten densest cities in the US. Moreover, seven are in the top ten for traffic congestion, as measured by the TTI’s Urban Mobility Report 2012.

In terms of the policy implications of his work, Prof Levinson concludes:

There are many ways to make transportation faster, some more viable than others. Adding capacity at bottlenecks, managing traffic flow effectively, and implementing peak road-user fees all would tend to increase road speeds. Adding connections in the transportation network would reduce the distances travellers must cover to reach their destinations.

On the land-use side, adding density depends on both market forces and public policy. In some cases market forces are constrained in the density they would provide, either due to zoning restrictions (height restrictions, maximum floor-area ratios, and so on) or minimum parking requirements. Similarly, the market responds to incentives. The tax code, which taxes buildings and land at equal rates, discourages construction

The Center hasn’t published data on other modes, or at least not yet. Since cars account for well over 90% of all motorised commutes in the US that’s not surprising.

Click - Rank of the accessibility to jobs by automobile for 51 U.S. metropolitan areas in 2010. Source: Center for Transportation Studies, Uni of Minnesota