While at first glance these two topics—transportation and urban form—might seem to be separate, in fact, they are inseparable. Transportation is often thought to be the field exclusively of the highway builders and the transit operators; while urban form is the domain of planning and real estate markets.
We start from the premise that cities are, as Jane Jacobs taught us, about bringing people together. Broadly speaking, the two features of cities that determine the ways in which people interact with one another are their urban form, the pattern and density of their land uses, and their transportation systems—the means by which people travel within metropolitan areas.
Moreover, these two are inextricably related: the transportation system shapes metropolitan form; and the form of the metropolis shapes the transportation system. Transportation mode shares are highly dependent on density. Below certain levels of density, many kinds of mass transit aren’t economically feasible.
Similarly, density is dependent on high levels of transit service. It is difficult to imagine how Manhattan (or any other similarly dense city) could exist with out its highly developed subway and bus transportation system. If, for example, car ownership in New York City were equal to that of the typical large US metro area, our calculations show that one would need a parking lot the size of Manhattan just to accommodate the additional cars .
Measures of transportation system performance that don’t consider urban form can be misleading or perverse. For example, one common measure—the travel time index—used to compare traffic congestion levels among cities ignores differences in travel distance, effectively rewarding cities with sprawling development patterns and long commutes .
Over time, investments in transportation provoke changes in land use patterns. Massive investment in the Interstate highway system (coupled with other subsidies to suburban development) fueled the highly decentralized development patterns observed in virtually every US metropolitan area after World War II. This led directly to the collapse of density in cities. Brown University economist Nathaniel Baum-Snow estimated that each additional highway constructed through a central city reduced its population by 18 percent.
When we build dense, compact cities, rich with mixed use developments, a high level of economic integration; we don’t need as much investment in transportation infrastructure, and the operating cost of transportation for households can be lower. The consumer savings can be substantial. In Portland—where residents drive almost 20 percent less than the national average, consumers save $1 billion annually on the cost of vehicles and gasoline—a “Green Dividend” that stimulates the local economy.
Portland shows that that there are metro-wide transportation cost savings to building compact, accessible communities. But virtually every metropolitan area has some neighborhoods—usually laid out using the traditional street grid, prior to World War II—that have higher density, a diversity of commercial and residential uses in close proximity, and a mix of housing types. Studies by Scott Bernstein and his colleagues at the Center for Neighborhood Technology show that the transportation cost savings in many of these neighborhoods help make urban living actually more affordable to households.
The urban core is the nucleus of the metropolitan area and its health is vital to regional economic success. Certain functions happen best and only in dense cores. Urban services and social function are better in cities with strong cores.
A healthy urban core reinforces the success of a regional economy. Cities with dense, economically diverse, close-in urban neighborhoods play key roles in assimilating immigrants, making transit work better, providing affordable housing, promoting economic opportunity, strengthening civic participation and reducing the emission of greenhouse gases. A struggling or unattractive core is a liability to the entire metropolitan area.
In short, metropolitan areas are not formless blobs. Having a vital urban core is essential to the effective functioning of metropolitan areas. The geographic shape of a metropolitan economy matters greatly to its success and efficiency. A sprawling “pancake” metropolitan area imposes high costs on its citizens for infrastructure and travel costs and produces greater economic segregation. A “donut” metropolitan area—one with a weak center—can’t achieve the critical mass needed to drive economic success.
There are a variety of ways to define the urban core. Individual cities often have their own local, traditional definitions of what constitutes the downtown. Many studies compare values for the “central city”—typically the largest municipality in a metropolitan area—to the overall values for the metropolitan statistical area. But, municipal boundaries are a poor choice for making comparisons among metropolitan areas because central cities vary substantially across metropolitan areas. Some central municipalities account for a majority of their metropolitan area’s residents and include some areas that would be commonly thought of as suburban, while central municipalities are less than 20 percent of a region’s population. Consequently, following an approach developed by Ed Glaeser, we define the urban core as the area within three miles of the center of the central business district.
Census tracts—neighborhoods averaging about 4,000 persons—are the key building blocks for estimating economic and demographic characeteristics of the nation’s urban cores. The aggregated five-year totals of the American Community Survey (ACS) provide sufficient sample size to make reliable estimates at the Census Tract level. Analysts use this ACS data and Geographic Information System (GIS) software to estimate values inside the three-mile ring drawn around the center of the central business district of the most populous city in each metropolitan area. Because the data are drawn from surveys fielded over five years, they do not reflect the values for any particular year, but rather represent the average level of each value over the five-year period.
One key marker of a strong urban core is the educational attainment of its residents. The educational attainment of the urban core plays a disproportionate role in determining the educational attainment of the metropolitan area. Richard Florida’s analysis shows those metropolitan areas with the biggest education differentials in favor of the urban core have the highest overall levels of metropolitan educational attainment. Conversely, those areas with the weakest cores, relative to their suburbs, have the lowest levels of metropolitan educational attainment .
The four-year college attainment rate is a benchmark measure of talent. This indicator counts the fraction of those age 25 and older, who have completed at least a four-year college degree. Metro areas vary widely on this measure. Fewer than nine percent of urban core residents in Las Vegas have completed a four-year degree, compared to more than 65 percent of those living in New York’s urban core. Overall, educational attainment is typically lower in the urban core than in the mtropolitan area, but two-fifths of all metropolitan areas have higher education attainment in close-in urban neighborhoods. New York and Chicago have much higher educational attainment in the urban core (180 percent and 201 percent higher than their surrounding metro areas, respectively). Portland, Seattle and Atlanta also have substantially higher levels of educational attainment in the urban core than in the remainder of the region. Several cities have relatively very low levels of educational attainment in the urban core. Las Vegas and San Antonio have college attainment rates in the urban core that are, on average, less than half those in their metro areas.
The size of a region’s population living in the urban core has a major influence on the transportation demand in a metropolitan area. Urban cores have the highest levels of walkability, the shortest average commute trips, and the highest levels of transit accessibility. In cities, we have seen declining vehicle miles traveled- or VMT- decline. To see more information about this as well as other information on congestion costs, go here.
Walkability maps compiled by Walk Score, show that residents living in urban cores have many more common destinations located within walking distances of their homes.
New transit accessibility maps prepared by David Levinson and his colleagues at the University of Minnesota show that residents of close in urban neighborhoods in every metropolitan area have the highest levels of accessibility to jobs via transit.
Because so many destinations are close at hand, and they have good access to jobs, central city residents tend to have much shorter average commute times than other metropolitan resident. (See the interactive map here)
