Answer: just 15 percent, or about one out of seven trips.
And according to this (somewhat dated) study by the Puget Sound Regional Council, even during the peak afternoon rush hour, only a minority of trips actually take us from work to home. We make lots of different kinds of trips during that time—shopping, school, errands, social visits, and the like.
Still, the issue of commuting—particularly, how to make it faster, and how to accomodate more drive-alone commuters—dominates transportation planning decisions, probably well out of proportion to its importance. But perhaps the real place to start isn’t with the commute itself, but with the places where we live. Improvements in community design that let us drive less to go shopping, or to drop the kids off at school, could make a bigger difference to our commutes than highway projects specifically aimed at increasing traffic capacity.
Just a thought…