Marchetti constant: Anthropological Invariants in Travel Behavior, and Zahavi's UMOT

"Marchetti's constant is the average time spent by a person for commuting each day, which is approximately one hour." [Source Wiki].

Zahavi "fundamental equation of travel demand": 

M / T = v . c

where M is travel-money budget, T is daily travel-time budget, v is mean daily speed, and c is average cost per unit distance traveled. In words:

What individuals are willing to spend in money (M) and time (T) = product individual would like to purchase from system supply in its performance (v) and price (c) of using it.

Explanation: if travel system is slow, then, people exhaust travel-time budget long before the travel-money budget. This disequilibrium is solved by buying faster travel (e.g. a car, or more cars)

References:

Marchetti paper Here

Zahavi's report here and here.