Be honest: when you're factoring how long it takes somewhere, you sometimes forget that parking your car is actually a part of the equation. This is especially an issue if you live in a giant metropolitan area, which is why Google has introduced a parking difficulty featurette to Maps.
Now when you look for directions or driving time to your Friday soiree, Maps will factor in the usual parking time in that particular area based on historical data. Unfortunately, the feature is only available in 25 areas right now. They are: San Francisco, Seattle, Miami, Atlanta, Boston, Charlotte, Chicago, Detroit, Los Angeles, Minneapolis/St. Paul, New York City, Orlando, Philadelphia, Pittsburgh, San Diego, St. Louis, Tampa, Washington, DC, Cleveland, Dallas/Fort Worth, Denver, Houston, Phoenix, Portland, and Sacramento.
The parking ability will arrive in a Google Maps software update on your mobile device.