Much of the theoretical literature assessing the effects of urban population and employment decentralization on commute length or travel behavior is based on the monocentric model (Alonso, 1964; Wheaton, 1979). In that model, the market for urban land is defined by the inelastic demand for a commute to the city center by one-worker households. Individual households choose where to live by trading off the value of access to their current jobs against the cost of housing, resulting in declining equilibrium housing rents and residential development densities as commute length increases (i.e., with increasing distance from the city center). Transportation costs thus determine rents, and the extent of decentralization is explained by the interaction of transportation costs with household preferences, given their financial and time budget constraints. In the end, suburban residents drive more but are compensated by lower rents per unit of land.
Simple polycentric elaborations on the monocentric model permit workers to choose from among several job locations within a given metropolitan area, allowing greater average proximity between home and work (White 1976, 1988; Wieland 1987; Yinger 1992). In a world where employment exogenously disperses, this basic polycentric version of the monocentric model suggests that commutes should get shorter, ceteris paribus.1 Recognizing that wages may play a role in compensating for the journey to work, in addition to land rents, increases the likelihood that firms face similar incentives (McMillen and Singell, 1992; Wheaton and Sivitanidou, 1992; Macek, et al., 2001; Senesky, 2001; Wheaton, 2002).
Gordon, Kumar and Richardson (1989b) provided one of the first empirical tests of the hypothesis that employment decentralization might reduce commuting, in this case commute duration. They used Census and satellite land use data for 82 U.S. metropolitan areas to investigate the relationship between mean commute travel time by auto or transit and measures of urban form: total urban land area, proportion of workers in industrial and commercial employment respectively, and net land use density by type, controlling for area-wide average income.
In regressions on commute duration, the authors found that spatially larger cities had longer commutes, while shorter commutes were associated with a higher proportion of industrial employment. Higher residential density was associated with commutes of longer duration. The proportion of employment in the central city was highly associated with commutes of longer duration. The authors concluded that both residential and employment dispersion lead to shorter commutes.
In a descriptive study appearing the same year, the authors again investigated some of the same relationships between congestion and urban form, this time focusing on city size (Gordon, Kumar, and Richardson 1989a). They used the Nationwide Personal Transportation Surveys of 1977 and 1983-84, along with 1970 and 1980 Census journey-to-work data, to argue that larger cities do not necessarily have longer commutes because much of that growth occurred in suburbs. They make two main points in support of this argument. First, in 1977, durations of commutes originating outside central cities were roughly equivalent regardless of city size, but those originating within central cities were uniformly lengthier with increasing city size. This suggests that only suburban dwellers avoided the effects of increasing city size. (The 1983 results were not as clearly defined but tended to show the same general pattern.) Second, commute speeds were higher in suburban areas.
Subsequent analytical work has mainly focused on disaggregate data.2 Levine's 1990 dissertation and subsequent research (Levine 1990, 1998) explain residential location choice, in search of shorter commutes to suburban employment, as a function of housing supply as well as demographic factors. He specifically finds that the commutes of low-income households lengthen as employment suburbanizes due to shortages of affordable household near these jobs. Another example is Dubin's use of 1977 Baltimore data to estimate a reduced-form supply and demand model of the change in commute time (Dubin 1991). She found that men, non-Blacks, higher income households, and those owning their home enjoyed shorter commutes. Notably, even while commute distances increased for most, travel times fell, especially in the less congested suburbs.
To summarize, commute length is a complex phenomenon that is especially challenging to investigate empirically. Investigating the connection between commute length and employment or residential decentralization is no less challenging. On the one hand, employers may benefit from shorter commutes by being able to reduce wages. However, this may be mitigated by the need for workers to take into account future job locations. Other important influences on residential location, such as local municipal and district services, and accessibility to other nonwork activities, may also come into play as decentralization occurs. The equilibrium outcome is an empirical matter, where the empirical strategy must account for the endogeneity of land rents, wages, employment status, and car access. We also need good measures of other demand/supply factors, such as socio-demographic characteristics at the individual level.