One important way that conventional methods to ease car travel patterns (such as widening a road) can fuel negative consequences is through the concept known as the “travel time budget,” also known as Marchetti’s constant. The travel time budget refers to the amount of time humans are willing to allocate to travel on a regular basis. The settlement of American cities has always been limited by how far a family breadwinner is willing to commute to work, as transportation specialist S. B. Goddard has pointed out, a maxim that in our car-oriented society needs to include not just distance but time.
Cross-culturally and throughout history, people devote an average of about 1.1 hours per person per day to round-trip travel. African villagers, the middle class, the super-rich (who often travel by walking, personal automobile and airplane, respectively), all have similar travel-time budgets. This indicates travel homeostasis where improvements made to reduce travel time result in a compensatory change in behavior to maintain a constant travel time. This creates a vicious cycle where an increase in supply places more demand on the network, which triggers transportation professionals to increase the supply (for example, by widening a road). As a city’s transportation system expands to allow longer and higher-speed travel, people will disperse in a pattern that in the long run will return to that 1.1-hour round-trip commute equilibrium.
By designing only for car travel—with huge roads, huge intersections, huge parking lots, huge speeds and huge subsidies, cities thus unknowingly dictate how they would develop when they deployed such elements. The immediate goal became making motorists happy, and drivers always want less traffic congestion, shorter drive time, fewer traffic signals, and higher speed limits. It has become a vicious cycle that locks cities into sprawl and its attendant ills—worse congestion, malls, strip retail stores, seas of parking lots, and on and on.