I also received this note from a regular reader:

Based on review of 16 mostly American and European studies of cruising conducted between 1927 and 2001, Shoup concludes that cars searching for free parking contribute to over 8% of total traffic. The relevant New York City study was conducted in 1995 by John Falcocchio, Joe Darsin and Elena Prassas.  They concluded the average time drivers took to find a curb space between 8 and 10 a.m. was 7.3 minutes, increasing to 10.6 minutes between 11a.m. and 2 p.m.  According to their research, cruising for curb parking created about 8 percent of the total vehicle miles traveled in west Midtown.

Read more at the Tri-State Transportation Campaign

Also, I forgot that AD over at Starts and Fits did a great summary of Shoup's work on the lure of free parking and the inherent inequity to non-drivers:
The interesting thing about our autotopia is that as everyone pays for parking except the person using it. For example, shoppers and office workers pay for spaces mandated by the municipality regardless of whether they drive to their store or office or walk or take the bus or train. Shoup figures that 99% of all parking is free to the user. If you ride your bike to the Wal-Mart (imagine that!), part of the money goes to pay for the parking spot you did not use.