A few weeks ago, a new Parking Difficulty icon started showing up in Google Maps 9.44 beta in some cities in the United States, then Google officially announced it and specified where it's available. It also said that the estimate is "based on historical parking data," similar to how traffic and popular times and visit durations are calculated, but it didn't go into a lot of details. Now those details are further clarified in a post on the Google Research Blog.

The difficulties of calculating parking availability stem from the many, many factors that can influence the equation: time and day and weather and holidays/events, permit or illegal parking in park-meter areas, vacant spots with paid park-meters from cars that left early, parking lots with multiple levels and different structures, and so on. Even with these issues accounted for, there's the false data coming from private/gated parking spots, taxis dropping users who look like they immediately found a parking and reached their destination, and bus stops showing up in the data sets as popular parking spots.