National household travel survey trip distances, 2009

Half of all trips in the U.S. are four miles or less
40% of all trips are U.S. three miles or less
These stats are handy and important for people supporting better places for bikes as they make the case that there's a lot of potential for shifting short trips. But it's hard to find the data behind the numbers. They came from the most recent available version of the National Household Transportation Survey, from 2009.
To find these numbers, run these data tables by trip length using only reported mileage (the last column).

Add them up and you get:

49.54% are four miles or less
40.37% are three miles or less

Or use this handy spreadsheet below: