The Pigeon

Two friends decide to get together; so they start riding bikes towards each other. They plan to meet on the halfway. Each is riding at 6 MPH. They live 36 miles apart. One of them has a carrier pigeon, that  starts flying as soon as the friends start riding. The pigeon flies back and forth at 18 MPH between the 2 friends until the friends meet. You must presume that the pigeon can go from 0 to 18 mph instantaneously, and it also turns around instantaneously.

How many miles has the pigeon covered?