PREDICTIONS – 10 Years Later - Santa Fe Institute

9. REACHING THE CEILING EVERYWHERE
This invariant can be combined with another universal constant, the
time available for traveling. Yacov Zahavi studied the equilibrium between
travel demand and supply, and urban structure. 11 He and his
collaborators showed the following: (a) Inhabitants of cities around the
world devote about one hour and ten minutes per day to urban transport,
be it by car, bus, or subway. (b) For this transport they all spent a rather
constant percentage of their income, 13.2 percent in the United States,
13.1 percent in Canada, 11.7 percent in England, and 11.3 percent in
Germany. (c) What varies from one individual to another is the distance
they cover; the more affluent they are, the further they commute. Zahavi
concludes that whenever there is a real discrepancy between these constants
and a person’s behavior, the natural balance is disturbed and
tensions and rejection might ensue.
Coming back to the car, at an average speed of thirty miles per hour,
a traveling time of one hour and ten minutes a day translates to about
thirty-five miles. This daily “quota” for car mileage is indeed corroborated
from data on car statistics. 12 During the last fifty years, yearly
mileage in the United States has been narrowly confined to around
ninety-five hundred miles, despite the great advances in car speed and
acceleration over this period. It turns out to be 36.4 miles per working
day, in good agreement with the earlier result.
The long-term stability evidenced by the above invariants is proof of
a balance between time available, disposable income, and road networks.
The average distance covered per day becomes the natural
limiting factor in defining the size of urban areas. Communities grow
around their transport systems. If it takes more than seventy minutes to
get from one point to another, the two points should not reasonably belong
to the same “town.” Cars permitted towns to expand. When people
only traveled on foot, at three miles per hour, towns consisted of villages
not much larger than three miles in diameter.
There was a factor of ten in speed between foot and car transport,
but also between car and airplane transport, taking the average airplane
speed as around three hundred miles per hour. Airplanes expanded the
limits of urban areas further, and it is possible today to work in one city
and live in another. Air shuttle services have effectively transformed
pairs or groups of cities in the United States, Europe, and Japan into
large “towns.”
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