The largest proportion of commuting in U.S. metropolitan areas occurs from suburb-to-suburb, standing at 40 percent of metropolitan commuters as of 2015. The following chart presents data from 2015 on America’s metropolitan commuting patterns:
This trend reflects that as the American population has moved to metropolitan suburbs over the past several decades, many of their jobs have followed. In other words, the “jobs-housing balance,” the distribution of employment opportunities and worker population in a geographic area, has readjusted for the long term, and commuting patterns express the new realities. This article will explore these trends, by showing the data on long-term shifts of population and jobs and presenting mathematical mechanisms through which these shifts are translated into fuel consumption.
As the American population has moved to metropolitan suburbs over the past several decades, many of their jobs have followed.
Understanding this background can help to better understand the development of fuel-consumption patterns over the next several decades, particularly as 86 percent of Americans now live in metropolitan areas, and as this number is likely to either hold steady or grow. Moreover, the Census Bureau predicts continued population growth of 20 to 25 million Americans per decade, indefinitely, which means 80 to 100 million new Americans every fifty years. As population increases, how will it affect commuting patterns and fuel consumption?
Some urban planning groups have been advocating higher-density, more compact, more walkable urban design, whereas developers and other groups have continued to add to sprawl by building low-density buildings and neighborhoods. The outcome of these long-term conflicts could affect the jobs-housing balance and in turn the distances people travel from home to work and the amount of fuel they consume. How should we make sense of the coming changes?
History: Suburban population growth and jobs-housing balance
This situation of predominant suburb-to-suburb commuting came into existence gradually, as Americans moved away from cities in the well-known “flight to the suburbs” that began in the 1950s and has continued for decades. As the residential population of suburbs grew into the largest population of any land-use type (city, suburb, rural) in the United States, enormous pressure was applied for jobs to move out to the suburbs as well. In turn, this created a new jobs-housing balance that can be seen in today’s commuting patterns, where people who both live and work in suburbs are the largest proportion of America’s metropolitan population. The explosive growth of the suburban population can be seen in the following chart:
In 2010, with 158 million people residing in the suburbs of metropolitan areas, versus 101 million people residing in the central city of metropolitan areas, 61 percent of metropolitan residents lived in suburbs. Therefore, the beginning commute point for each of these persons was “suburb.” Interestingly, most metro-area jobs (52 percent, as of 2015) are still located in central cities, but a high enough proportion of metro-area jobs are located in suburbs (48 percent in 2015) to render “suburb” the most common end commuting point for metro-area workers. Consequently, “suburb-to-suburb” becomes the most common beginning-to-end, point-to-point commute for residents of U.S. metropolitan areas. The distribution of jobs relative to people can be seen in the following chart, which shows 2015 data on the so-called jobs-housing balance for America’s metropolitan areas.
As future patterns develop over time, the jobs-housing balance will be a key variable to track.
As future patterns develop over time, the jobs-housing balance will be a key variable to track. A broader inquiry, beyond the scope of this article, will be necessary in order to fully understand the complex determinants of where Americans choose to live and where they choose to work. Nevertheless, the following section sketches fundamental mathematical calculations of how data on population and geography can translate into concrete demand for motor fuel consumption.
Translating geography into fuel demand
To understand the mechanics of how geography translates into fuel demand, we begin first with a simple but comprehensive equation for fuel consumption:
The top line says that fuel consumption increases either the further that people travel or the less fuel-efficient their vehicles are. The second line breaks out these variables further. In the numerator, vehicle-miles-traveled (VMT) result from either the number of trips or the length of those trips, so taking more trips or longer trips will increase fuel consumption, everything else being equal. In the denominator, fuel-efficiency is measured in the well-known indicator of miles-per-gallon: the higher a vehicle’s miles-per-gallon, the more fuel-efficient and the lower the fuel consumption, everything else being equal (and vice versa for lower miles-per-gallon).
Several questions can be asked about how commuting patterns relate to the variables in the fuel consumption equation, within the context of metropolitan geography. These are important factors to consider in assessing the impact of the jobs-housing balance upon specific fuel consumption levels, although definitively answering these questions is beyond the scope of this article.
- Trip Length: Are suburb-to-suburb commutes longer than other commuter trips?
- Road-Type and Fuel-Efficiency: Do commuters drive on highways or local roads? For each type of road, how do typical speeds and driving patterns affect fuel efficiency?
- Traffic Congestion and Fuel-Efficiency: If suburb-to-suburb commutes lead to congested roads, does this reduce fuel-efficiency?
Interestingly, fuel-efficiency shows up again and again. If cars were dramatically more fuel-efficient, it would not matter how long the trips are or how frequent, because fuel consumption would be greatly reduced anyway. Thus, the rate of vehicle fleet turnover, an important topic addressed here, must be taken into account in assessing urban geography, commuting patterns, and fuel consumption.
Energy security implications, the long-term perspective
Between 2015 and 2060, the Census Bureau projects U.S. population growth of approximately 100 million. Will these 100 million new Americans both live and work in suburbs? Will suburb-to-suburb remain the most widespread commuting pattern in 2060? On one hand, urban planning groups may attempt to relocate jobs and homes to compact, walkable cities. On the other hand, developers may continue building low-density housing subdivisions, office parks, and strip malls. Which way will the balance tip: Compact cities or sprawling suburbs?
If a large number of people continue to live in suburbs, jobs will continue to locate in suburbs, and suburb-to-suburb commuting levels will endure or even increase.
Bottom-line: Jobs follow people. If a large number of people continue to live in suburbs, jobs will continue to locate in suburbs, and suburb-to-suburb commuting levels will endure or even increase. Those interested in U.S. oil dependence and transportation demand should monitor the jobs-housing balance and its impact on suburb-to-suburb commuting.