30% Cruising rate isn’t really accurate…. Don Shoup

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30% Cruising rate isn’t really accurate…. Don Shoup

It seems that the discussion about the 30% cruising rate is continuing. Here is an email stream between UCLA’s Don Shoup and Steven Polzin, Director, Mobility Policy Research, Center for Urban Transportation Research, University of South Florida.  Highlighting is mine.

First Shoup starts the discussion:

Hi Steve,

I agree with what you say about the 30 percent in your Planetizen post.

http://www.planetizen.com/node/87288/playing-telephone-transportation-data

I appreciate that you mentioned the “appropriately qualified data in the original paper.” Unfortunately, many people who have never read The High Cost of Free Parking often think I said that 30 percent of city traffic is cruising for parking.

I did summarize the results of 16 studies of cruising in 11 cities on four continents.  Researchers found that 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.  For the 16 studies the average share of traffic that was cruising was 30 percent and the average search time was 8.1 minutes.

As I explained in the book, the data in these studies, which date back to 1927, 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. The studies were selective because researchers measured cruising only when and where they expected to find it—where curb parking is underpriced and overcrowded. Nevertheless, 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 time and fuel for many decades.

On most streets at most times, no one is cruising. But many people want a number, and I can’t stop anyone from saying that 30 percent of traffic is cruising. Nevertheless, on busy streets where all the curb spaces are occupied and traffic is congested, a substantial share of traffic may be cruising.

For example, when researchers interviewed drivers who were stopped at traffic signals in New York City, 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. This doesn’t mean, however, 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.

On a congested street where all the curb spaces are occupied, one simple way to estimate how much of the traffic is cruising is to observe whether the first car that approaches a newly vacated space parks in it. If, for example, the first or second driver who approaches a newly vacated curb space always parks in it, this suggests that most of the traffic is cruising for parking.

An even simpler and quicker (though perhaps less humane) way to sample the traffic flow is to approach the driver-side door of a car parked at the curb with a key in your hand, as if to open the door. If the first driver to see you with a key apparently poised to unlock the door always stops to wait for the space, most of the traffic is probably cruising. The stopped car blocks a lane of traffic just like a double-parked car. Unfortunately, you must then use body language to suggest that you have changed your plans and have decided not to leave, regrettably disappointing the driver who expected to park in the space. If you do this several times, and the first or second driver to see you with a key in your hand always stops to wait for a space, what share of the cars in traffic would you think are cruising? When I did this on Pike Place in Seattle, the first driver who saw me with a key in my hand always stopped traffic to wait for the space.

Because most streets usually have some vacant curb spaces, the share of traffic that is cruising on most streets is probably zero. Because curb parking is underpriced and overcrowded in the busiest parts of most of the world’s big cities, however, the sun never sets on cruising.

Don Shoup

Steve Responds:

Thanks for getting in touch. As you can tell, my frustration was that people have generalized your results to “cities” leaving off reference to time, location, and geographic scale over which the numbers could appropriately be applied. It was pretty obvious to me that the 30% number was incidental to your work but the number that folks latched on to. In light of the nature of the data it would’ve been perhaps better to not calculate or show a simple average.

Regarding current trends in cruising for parking, my perception is that in many areas the fundamental nature of how we provide parking now versus decades ago could be changing the extent of cruising. Much more parking is in structures integrated with destinations and less appears to be on the street. Perhaps some of that cruising is circulating within garages. I don’t follow this body of data but my guess is variation in context is huge. In many of the newer cities, Dallas, Houston, Tampa, Atlanta, etc. that developed primarily during the auto era I suspect the extent of cruising for parking is very limited.  If I had to guess I would suspect that cruising for parking is less than a percent of total national vmt. Also curious as to what share of VMT used to be spent by people looking for a location before we had GPS?

Steven E. Polzin, PhD

polzin@cutr.usf.edu

And Don responds:

Hi Steve,

Thanks for your message and for your post in Planetizen questioning whether 30 percent of traffic in cities is caused by drivers looking for parking.

I agree that most cruising occurs in older areas were built before cities invented minimum parking requirements. Since the 1950s most cities have required so much off-street parking for any new development that few drivers need to cruise to find a free space. Cities have “solved” the problem of cruising for on-street parking by requiring plenty of off-street parking everywhere. As a result, cruising as a share of total VMT in the US must now be very small. Nevertheless, cruising as a share of VMT in areas built before minimum parking requirements can still be very high.

Minimum parking requirements, however, are an extraordinarily expensive and damaging way to reduce cruising. Minimum parking requirements are propagated by pseudoscientific data that come from Institute of Traffic Engineers’ publication, Parking Generation. Unlike a careless statement that 30 percent of traffic is cruising for parking, the ITE’s parking generation data have done enormous damage, as I tried to explain here and here and here.

If researchers want to delve into transportation numbers with dubious pedigrees, they have a target-rich array of data to debunk, starting with the ITE’s Parking Generation and Trip Generation.

I appreciate your calling attention to the frequent claim that 30 percent of city traffic is cruising for parking. I agree with your explanation that newer parts of cities have much less cruising because they provide so much off-street parking.

I hope more transportation academics will follow your lead and challenge other flaky transportation numbers, especially the ITE’s parking and trip generation data. Placing unwarranted trust in the accuracy of uncertain data can lead to disastrous policy choices.

Don Shoup

John Van Horn

John Van Horn

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