Marchetti's constant is the average time spent by a person for commuting each day, which is approximately one hour.
According to Marchetti, although forms of urban planning and transport may change, and although some live in villages and others in cities, people gradually adjust their lives to their conditions (including location of their homes relative to their workplace) such that the average travel time stays approximately constant.
Ever since Neolithic times, people have kept the average time spent per day for travel the same, even though the distance may increase due to the advancements in the means of transportation.
Each improvement in locomotion has increased the area over which people are compelled to move: so that a person who would have had to spend half an hour to walk to work a century ago must still spend half an hour to reach his destination, because the contrivance that would have enabled him to save time had he remained in his original situation now—by driving him to a more distant residential area—effectually cancels out the gain.
A related concept is that of Zahavi, who also noticed that people seem to have a constant "travel time budget", that is, "a stable daily amount of time that people make available for travel."