Is 30 Percent of Traffic Cruising for Parking?

February, 2019

A number that is cited often enough can become plausible even when no one knows its pedigree. For example, I often read that 30 percent of cars in city traffic are cruising for curb parking. Here are some published examples:

“Studies have found that an estimated 30 percent of all drivers currently on the road are actively looking for parking.”

“Researchers found that 30 percent of emissions from road transport are caused due to traffic congestion instigated by people searching for parking.” 

“It is estimated that nearly 30 percent of urban congestion is created by drivers cruising for parking.”

“The literature contains estimates that the proportion of cars traveling on downtown city streets during the business day that are cruising for parking is 30 percent or even higher.”

Even if the cruising share is not 30 percent, it is worth worrying about.

Where did this 30 percent rumor come from? Apparently from me. In The High Cost of Free Parking, published in 2005, I reported the results of 16 studies of cruising for curb parking in Chapter 11:

“Table 11-5 summarizes the results of 16 studies of cruising in 11 cities. Between 8 and 74 percent of traffic was searching for parking, and it took between 3.5 and 13.9 minutes to find a curb space. But these studies dating back to 1927 are mainly of historical interest. The data were probably not very accurate when they were collected, and the results depended on the time of day, the specific place, and the season when the observations were made. Still, cruising today is similar to what drivers have done since the 1920s, and the studies at least show that searching for underpriced curb parking has wasted an enormous amount of time and fuel for many decades. The studies are selective because researchers study cruising where they expect to find it—on streets where curb parking is underpriced and overcrowded. But because curb parking is underpriced and overcrowded in the busiest parts of most of the world’s big cities, the sun never sets on cruising.”

I pointed out that the cruising shares ranged from 8 percent to 74 percent and I did not mention the average of 30 percent. I said the research referred to busy streets where curb parking is underpriced and overcrowded. Nevertheless, some readers applied the 30 percent to all traffic, perhaps as a shorthand way to suggest that cruising is worth worrying about.

In Chapter 25 of Parking and the City, published in 2018, I reported five new studies of cruising and emphasized that the data do not describe all traffic: 

“Cruising creates a mobile queue of cars waiting for curb vacancies, but cruisers are mixed with traffic that is going somewhere, so no one can see how many cars are in the queue. Nevertheless, a few researchers have analyzed cruising by videotaping traffic flows, driving test cars to search for a curb space, and interviewing drivers who park at the curb or are stopped at traffic lights. Twenty-one studies of cruising behavior were conducted between 1927 and 2011 in the central business districts of 13 cities on four continents (Table 25-1). The average time it took to find a curb space was 7.5 minutes, and on average 34 percent of the cars in the traffic flow were cruising for parking. 

“For example, when researchers interviewed drivers who were stopped at traffic signals in New York City in 2007, they found that 28 percent of the drivers on one street in Manhattan and 45 percent on a street in Brooklyn were cruising for curb parking. These estimates cannot, of course, be extrapolated to suggest that 28 percent of all traffic in Manhattan is cruising for parking or that 45 percent of all traffic in Brooklyn is cruising for parking.

“The results in Table 25-1 do not suggest that 34 percent of all city traffic is cruising. On a street where some curb spaces are vacant or where curb parking is not permitted, no cars will be cruising. No cars on freeways are cruising for parking. On a street where no curb spaces are open, however, many of the cars in the traffic flow may be cruising. 

“Even if a high share of traffic is cruising, the number of cruisers will be low if the traffic flow is low. For example, at midnight on a street where all the curb spaces are occupied but the traffic flow is only two cars an hour, few cars will be cruising even if they are 100 percent of the traffic flow. But where all the curb spaces are occupied and traffic is congested, many cars can be cruising for parking. 

“The share of traffic that is cruising can change from one minute to the next because cruising is a variable, not a constant. Cruising can vary regularly by location and time of day, just as the volume of traffic does. There can be an average share over the day, but that average doesn’t predict the share of traffic that is cruising at any particular time or location.”

Cruising is variable but predictable. Figure 1 shows the results Jin Cao, Monica Menendez, and Rashid Waraich found when they examined cruising for parking in central Zurich. From noon to 3 pm, when all the curb spaces were occupied, 50 percent to 70 percent of the traffic was cruising for parking. Open curb spaces were available at other hours and few cars were cruising. At all hours, the cruising share varied from one minute to the next. 

I can suggest a simple, though perhaps inhumane, way to measure the cruising share at any time. With a key in your hand, you can approach the driver-side door of a car parked at the curb. Holding your key at the door signals a soon-to-be-vacant curb space. If the first driver who sees you stops to wait for the space, some of the traffic is probably cruising. Unfortunately, you must then use body language to suggest that you have decided not to leave, disappointing the driver who expected to park in your space. 

Following this key-in-the-door insight, Robert Hampshire and I developed a statistical model to measure cruising by observing the number of cars that pass a newly vacated parking space before a car parks in it. Cleverciti supplied 876 observations recorded by its overhead cameras in central Stuttgart. Using the data, we estimated that 15 percent of the traffic was cruising for parking during the daytime on a Friday and Saturday in September 2017.

As more cities install traffic cameras, it will become easier to estimate the cruising share. Until then, if you had to guess what share of traffic is cruising on a busy downtown street with underpriced and overcrowded curb parking, what would you say? 

To educate your guess, you could use the key-in-the-door technique to sample the traffic. If no drivers stop when they see you at your car door, cruising is not a problem. But if the first driver stops to claim your about-to-be vacated spot, cruising is congesting traffic, polluting the air, wasting time and fuel, endangering cyclists and pedestrians, and accelerating global warming. Even if the cruising share is not 30 percent, it is worth worrying about.

REFERENCES

Jin Cao, Monica Menendez, and Rashid Waraich, “Impacts of the Urban Parking System on Cruising Traffic and Policy Development: the Case of Zurich Downtown Area, Switzerland,” Transportation, forthcoming.

Robert Hampshire and Donald Shoup, “What Share of Traffic is Cruising for Parking,” Journal of Transport Economics and Policy, Volume 52, July 2018, pp. 184–201.

Donald Shoup, “The Pedigree of a Statistic,” ACCESS, No. 11, Fall 1997, p. 41.

Donald Shoup, The High Cost of Free Parking, Chicago: Planners Press, 2005 and 2011.

Donald Shoup (ed.), Parking and the City, New York: Routledge, 2018.

Donald Shoup is Distinguished Research Professor
of Urban Planning at UCLA. He can be reached at
shoup@ucla.edu