This comes up from time to time e.g.

http://d-squareddigest.blogspot.com/2007/05/bits-and-pieces-congestion-pricing-road.html

and nobody has a cite.  There's a certain kind of fame that leads people 
people to invent attributions e.g. countless saccharine banalities 
credited to Albert Einstein.

Here's a more general history-of-economic-thought question.  Would 
Keynes have used "irrational" in this way?

Colin Danby