A new way to measure curbside congestion.
On a congested city street where all the curb spaces are occupied, some of the traffic is probably searching for curbside parking. This cruising for parking creates a moving queue of cars waiting for vacancies but it is difficult to know how many cars are in the queue because the cruisers are mixed in with other cars that are traveling to destinations.
Cruising for parking stems from underpriced curb spaces. If prices are too low and no curb spaces are vacant, drivers searching for parking congest traffic, waste fuel, and pollute the air. Conversely, if prices are too high and many curb spaces are vacant, businesses lose customers, employees lose jobs, and cities lose tax revenue. Consequently, pricing for curb parking should follow the Goldilocks principle. The right price is the lowest price that keeps one or two spaces open for convenient access on every block so that any driver willing to pay will find a place to park.
Measuring cruising
How much traffic stems from cruising for parking? Table 1 summarizes the results of 22 studies of cruising in 15 cities on four continents, dating back to 1927. According to these findings, cruising for parking accounted for between 8 and 74 percent of traffic in the areas studied, and the average time to find a curb space ranged between 3.5 and 15.4 minutes. On average, 34 percent of cars were cruising, and the average time it took to find a space was eight minutes. However, these results do not represent all city streets because researchers tend to study cruising only where they expect to find it: on downtown streets where traffic is congested and all the curb spaces are occupied. Despite this selection bias, these studies do show that searching for curb parking has wasted time and fuel for decades.
Table 1. Cruising for Parking
Cruising is a variable, not a constant. For example, a study of traffic in central Zurich found that the share of cars cruising varied between 20 and 70 percent from 11 a.m. to 4 p.m. There may be an average share of cruising traffic on a particular street throughout the day, but that average does not predict cruising at any particular time or location, and it certainly does not apply to a whole city. These averages do not suggest that a third of all traffic is cruising for parking. On streets with plentiful open curb spaces, no cars are likely to be cruising. The share of traffic that is cruising can also change from one minute to the next, just as traffic volumes shift throughout the day.
Understanding how much traffic is caused by cruising for curb parking is important because new demands are overloading the curb. The growth of e-commerce has increased the demand for loading zones. Uber and Lyft have increased the demand for curb space to pick up and drop off passengers. Traffic congestion has increased the demand for dedicated bus lanes. Cyclists want bike lanes and pedestrians want wider sidewalks. The curb is the new urban frontier, and parking may no longer be the most productive use of this space.
A new way to estimate cruising traffic
The studies in Table 1 used three main methods to study cruising for parking. They either observed cars in the traffic flow (Detroit, Freiburg, Los Angeles), interviewed drivers who had parked at curbs or were stopped at traffic lights (Barcelona, Brisbane, Cape Town, Los Angeles, New Haven, New York), or conducted park-and-visit tests (Cambridge, London, New York). Unfortunately, these research methods are labor-intensive, time-consuming, expensive, and hard to replicate.
We propose a simpler way to estimate the share of traffic that is cruising for parking on a congested street where all of the curb spaces are occupied. We do this by observing how many cars pass a newly vacated space until a driver parks in it. If, for example, the first or second driver who approaches a newly vacated curb space always parks, it suggests that most of the traffic is cruising for parking. But if many cars pass by before one takes the vacated space, we can assume that most of the traffic is not cruising.
To determine the amount cruising for parking using this method, we employ a probability distribution where each observation of cars in traffic has only two possible outcomes: parking or passing. In a large sample, if the first driver who approaches a newly vacated space always takes it, all of the traffic is probably cruising. If an average of three cars pass the space before a car takes it, then about a third of the traffic is probably cruising. And if an average of 10 cars pass before a car takes it, then 10 percent of cars are probably cruising.
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