In the 1950s, the New York subway faced a problem that will be familiar to users of public transport all over the world.
At peak times, it was overcrowded; at other times, the trains were empty.
The mayor commissioned a report, which concluded the problem was subway riders paid a flat fare. No matter where you boarded, how far you travelled, or when you made your trip,
it would cost you 10 cents.
Might there be some more sophisticated approach? Perhaps so. The report's foreword singled out a proposal from one of the 17 authors, economist William Vickrey.
"The abandonment of the flat-rate fare in favour of a fare structure which takes into account the length and location of the ride and the hour of the day is obviously a sensible step provided the mechanical problems involved can be solved," he said.
Vickrey's basic idea was simple: when the trains were busy, charge more. When they were quiet, charge less.