From the 2011 NALC Postal Record:
The Postal Service has a long history of using bikes for city delivery. The Postal Service estimated recently that the carriers on bikes in nearby Sun City, AZ, alone save 7,800 gallons of gasoline a year, which adds up to roughly $27,300 at todays prices for gasoline.
Some other countries still rely on bicycles as a significant part of their postal delivery systems. Urban residents of Denmark and Holland get much of their mail delivered on two wheels. In New Zealand, 80 percent of letter carriers deliver using a bike. But changes like automation, and the longer routes that resulted, caused the U.S. Postal Service to drop bikes in favor of trucks. Today, bike routes are found only in areas with warm weather and flat terrain like Arizona and Florida.
Letter carriers deliver on foot, via LLVeven by bike: If youre a carrier, whats your mode? goo.gl/vUBP2c
Letter carriers deliver on foot, via LLVeven by bike: http://t.co/i9LkXYaBtV If youre a carrier, whats your mode? pic.twitter.com/nW2Q5aSIGp
Letter Carriers (@NALC_National) June 2, 2014