The Shoup Dog read my blog below and responded — Here it is:
I sympathize with your friend who doubted that 30 percent of city traffic is cruising for parking. Unfortunately, many people who have never read The High Cost of Free Parking often quote me as saying 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.
These studies date back to 1927. 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. 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 biggest cities, however, the sun never sets on cruising.