Urban Footprint


From the very beginning of civilization, transportation and land use have been inextricably linked. Up until the end of the 19th century, walking was the predominant form of transportation. Ergo our cities were compact and catered to travel by foot. It wasn’t until the beginning of the Industrial Revolution that innovations in transportation began stretching our urban footprints like fingers stretching the edge of pizza dough.

So how have transportation innovations impacted the urban form and why did they have that impact? Given what we know about driverless cars, how will they impact the urban footprint? Are there any policies that can be established to limit the negative impacts of this technology moving forward? These are the questions, this article hopes to answer. Driverless cars promise an advance in transportation no less significant than the car itself and they may be just as impactful.

Transportation and Urban Form

Peter Mueller famously mapped the urban footprint impacts of the various transportation eras in his paper titled “Transportation and Urban Form: Stages in the Spatial Evolution of the American Metropolis”. What is clear from his research is that transportation innovations that make travel cheaper, in terms of time and/or money, expand the urban footprint. This was true with the street car, the car and the freeway. But he draws a critical distinction between Europe and America. In American, central cities were hollowed out as transportation technology improved. In Europe, central cities retained their relative importance. He attributes these different experiences to land availability (America-high; Europe-low), land fragmentation (America-low; Europe-high) and, most importantly, divergent public policies. American government at every level doubled down on policies that encouraged sprawl and supported car-centric development. Europe, for the most part, did the opposite.

Transportation Eras

Travel Time Budget

In the 1970’s Yacov Zahavi began developing a theory that described a link between the urban footprint and transportation. What he found is that people are willing to spend between 1.1 and 1.3 hours traveling per day. Subsequent research has generally supported his theory but has found that travel time budgets vary by person. An individual’s travel time budget can be impacted by their income, cultural influences, age, etc.

Driverless Cars are anticipated to increase travel speeds and reduce the mental cost – which is arguably related to the time cost – of driving. Thus, all else equal, driverless cars will expand our metropolitan footprints, just as the car and the freeway did.

Travel Money Budget

People are clearly sensitive to the time it takes to travel somewhere, but they are also sensitive to the monetary cost of travel. When it costs more to drive a car, perhaps because gas prices have spiked or parking prices are high, people drive less. This is important to understand because driverless cars are likely to change the marginal costs of driving, that is, the additional costs of driving each mile. But how the marginal cost of driving changes depends on how we use driverless cars: will we own them as we own cars today or will access them like we do taxis as many have predicted.

If we own driverless cars, the marginal cost of driving may actually decrease. Many expect driverless cars to also be battery powered and electricity is cheaper on a per mile basis than gasoline currently. Driverless cars are also expected to drive more efficiently, which means cars will be able to go further on a single charge. If this happens, are urban footprints are likely to expand.

If we access cars as a taxi, however, the marginal cost of driving will likely increase significantly. When we own a car, we are really only paying for gas on a per mile basis. When we rent a car we are not only paying for gas, but also the purchase price, maintenance, insurance, etc. In this scenario, even if the total cost of driving decreases, we can expect an increase in the marginal cost of driving to translate to a reduction in overall driving, which, all else equal, may reduce the urban footprint.

Public Policy

As has been highlighted above, there is a very real chance that the driverless car will expand the limits of our cities, just as the car and freeway before it. But there are measures that cities can take to mitigate that risk. The two most effective methods are a user fee and a congestion fee. Rather than paying for roads with a hodge podge of property taxes, sales taxes, and gas taxes, they would be paid for by the people that use them in proportion with how much they use them.  This would have a direct impact on the marginal cost of driving and would encourage people to be more thoughtful when deciding how much to drive and how far to drive. In aggregate, these policies would help reduce the likelihood that driverless cars will increase sprawl.


When we look at the historical interplay of transportation and land use and the theories of travel time budget and travel money budget, it is evident that driverless car may very well expand the size of our cities. They are anticipated to increase travel speeds, reduce the mental burden of travel and they may reduce the marginal cost of driving if we end up owning them. Each of these factors would contribute to more sprawl. However, if we end up using these vehicles like we use a taxi that would significantly increase the marginal cost of travel and may actually reduce sprawl. Cities and states that hope to reduce the likelihood that driverless cars will contribute to sprawl would be well advised to employ a user fee and congestion mitigation fee, which, incidentally, would also solve ongoing transportation funding challenges.