American Scientist

Imagine a scene from the near future: You get dropped off downtown by a driverless car. You slam the door and head into your office or appointment. But then where does the autonomous vehicle go? It’s a question that cities would be wise to consider now. Self-driving cars may be on the roads within the next decade or two.

Automakers and specialized startups alike are aggressively developing automated vehicles (AVs), while government agencies explore ways to reduce regulatory barriers. Ride-hailing companies such as Lyft and Uber plan to operate some AVs, but others could become private robotaxis that drop owners off wherever they like and pick them up later.

Without policies to encourage sharing, it’s possible there could be many private AVs on the road. We are civil and environmental engineers who collaborated with Chris Hendrickson, director of Carnegie Mellon University’s Traffic21 Institute, to examine the potential effects of private AVs on cities.

Using Seattle as a case study, our analysis suggests that one of the biggest effects of AV technology may be on parking, as AVs leave expensive downtown spots behind in favor of cheaper parking outside the city center.

Parking’s Big Bucks

Parking takes up a lot of land in cities. Researchers at the University of California Los Angeles found that between 5 percent and 8 percent of urban land is devoted to curb parking. They estimated that the parking coverages—the ratios of parking area to total land area—in downtown Los Angeles and Houston are about 81 percent and 57 percent, respectively. A 2018 parking study conducted by the Mortgage Bankers Association found that Seattle’s parking density of 29 parking stalls per acre of land is twice its population density of 13 people per acre.

Because driverless cars could park outside urban cores to avoid the higher parking charges downtown, they might have a considerable effect on urban land use. And there are potentially big fiscal consequences. Many cities gather a substantial amount of money from parking-related activities, with the 25 largest U.S. cities collectively generating $1.5 billion in total revenue from parking fees and taxes in 2016.

In Seattle, for example, annual revenues from parking meters total about $37 million. In addition, the city collects $39 million in annual revenues from commercial parking lot taxes and $21 million from parking fines. Lower demand for parking could mean these funds—traditionally used for city operations including education, cultural resources, and libraries —would need to be replaced with other sources of revenue.

Modeling a City with Driverless Cars

To gauge the potential effects of private AVs on parking, we used Seattle as a case study, because data on all of its off-street parking lots are available. We looked at factors including energy use, emissions, parking revenue, and vehicle miles traveled (VMT), a key statistic used by traffic engineers to measure travel demand.

Our team obtained data from the Puget Sound Region Council on the daily occupancy and parking prices of all paid off-street parking garages and lots in downtown Seattle. We then identified areas outside of downtown with many unrestricted parking spaces, where vehicles can currently park free of charge during the day.

An AV would not become frustrated sitting in congestion or cruising to find an open curb space, whereas a human driver would.

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Then we computer-modeled privately owned AVs searching for cheaper parking, where each vehicle makes parking decisions based on availability and total cost, including both parking fees and all operational costs of the round trip. Each AV’s objective is to minimize cost. An AV would not become frustrated sitting in congestion or cruising to find an open curb space, whereas a human driver would.

We varied the operating costs of AVs across a range of values to understand how future changes in technology or an imposed per-mile tax might affect the results. And we considered a range of possible adoption rates for private AVs, starting from a point when they are exclusively available to a few high-income early adopters, to when they reach total market penetration.

At low penetration rates—where anywhere from 5 to 50 percent of all cars traditionally parked downtown become automated—AVs are usually able to obtain their choice of parking spaces. In most cases, these are in free parking zones closest to where they drop passengers off downtown.

With more AVs, these free parking spaces closest to the downtown area fill up, and cars must travel longer distances to obtain cheap parking. And as market penetration rates rise, each vehicle would travel additional round-trip miles in its quest for inexpensive parking.

With lower numbers of AVs on the road, this added travel would have a negligible effect on the total miles traveled by cars in the Seattle region. But if all private cars parking downtown were AVs, the total daily miles traveled would increase by about 2.5 percent, with each AV traveling on average an additional 8.5 miles per day. That change, even if it sounds small, could cause congestion along heavily traveled routes depending on the time, the day, and the mix of human-driven vehicles on the road.

Our simulation shows that there is enough free parking just outside downtown Seattle that AVs would no longer choose to park in downtown lots. At current prices, it’s more economical to travel for free parking than to park in a paid lot.

Some private AV owners may also rent out their car during the day as a ride-hailing service, but for others it might make financial sense to send their car home during the day and have it pick them up later, which would further increase overall vehicle miles traveled.

The End of Downtown Parking?

As AVs leave downtown, parking revenues could decline to the point where owning a parking lot or garage would no longer be economically viable. Cities could also lose a substantial amount of annual parking revenue in a future with more AVs. We see a few ways that cities could strategically adapt parking requirements to prepare for this possible future.

One option is congestion pricing: a fee or tax paid by users to enter the urban core of the city. That fee could also encourage more public and active transportation, such as biking and walking. Or cities could change the rules for parking in areas where it’s now unrestricted and free.

Cities could experiment with what’s called a scaled VMT tax: a fee for an AV to enter a downtown zone based on the number of miles it’s already traveled that day. This option might discourage an increase in housing sprawl and reduce the number of people using AVs to get downtown. In addition, encouraging the use of electric-powered AVs rather than gasoline-powered vehicles would reduce the environmental effect of any additional mileage.

Much of the land devoted to parking lots in today’s cities could be converted to parks, housing, or commercial spaces, and reducing curb parking could allow for wider bike lanes or sidewalks. To take advantage of changing parking demand, cities could build adaptable parking garages that can be converted to other uses if they’re no longer needed. Garages with flat floors and exterior ramps, rather than interior ramps, can more easily be transformed into commercial or residential spaces.

While robotaxis are not here yet, preparing now for changes in downtown parking and infrastructure will help cities respond when privately owned AVs start to hit the streets.