What Do Transportation Agencies Control?

Staff at transportation agencies, especially state DOTs, will emphasize that they oversee the transportation system, and don’t control land use. This is not strictly true in terms of direct oversight, and at best incomplete in terms of indirect impacts. Guerra et al. (2024) used detailed information on urban roadways nationwide to demonstrate that up to 25% of urbanized areas were dedicated to transportation (mostly roadway) infrastructure. Valued at trillions of dollars, the collective holdings of public transportation authorities like state DOTs makes them among the country’s most important real estate owners. 

Indirectly, DOTs also influence land use. Investments in roadway capacity, whether in the name of alleviating congestion or simply because a better route is available, create areas of changed accessibility. Land use changes follow, because residents and businesses rearrange themselves to take advantage of the connections. These in turn change long-run patterns of daily travel, typically consuming if not overwhelming the original increase in potential capacity. This is a form of induced demand, whereby the travel patterns change in response to transportation investments, mediated by the changes in accessibility (through both origins and destinations). In this pathway, additional transportation infrastructure drives eventual demand.

DOTs respond to ongoing land use change. In a stereotypical version, origins or destinations appear in the form of greenfield development, and then generate regional travel demand through new residents or opportunities, followed by political pressure for transportation systems to be altered to allow these new residents to realize the same level of access provided to others in a region. This is the familiar mechanism of suburban sprawl. Land use changes first, followed by the transportation system responding to new travel needs; transportation supply follows demand, through accessibility.

These pathways can co-exist, and are both mediated by accessibility, as depicted in Figure 1.3 . The relationship between land use and accessibility is reciprocal and complex. Increased accessibility can increase the desirability of one type of land use over another, and promote development of origins and destinations in areas of new access. Land use decisions increase or decrease access differently by modes. 

However, the sign and directionality of the transportation pathways are more straightforward. For the auto mode, investments in roadway infrastructure increase accessibility, through additional capacity, new connections, or increased speeds. The increased accessibility results in increased travel in the form of more, and longer trips, though this tendency is also influenced by a myriad of social and economic factors at the household and individual level. This additional trip making leads to capacity constraints, and congestion, which in turn feeds back to reduce the accessibility provided by the original infrastructure. In other words, accessibility is consumed by the travel behavior. At this point, additional investment is called for by residents who have experienced a loss of auto accessibility due to the increased use of the infrastructure, and the cycle begins anew.