[DoughtyOne]

Interesting premise Semper911... It would be advisable for them to transport a variety of ways to the release point, then see how close to the delivery method they retraced.

Checking out the route shown this time, it's clear the birds tried to use the ramps. Were those the ramps the delivery truck used? They just may be.

[FormerlyAnotherLurker]

Did they fly/drive the ramps in the wrong direction?

[DonQ]

Homing pigeons were being used LONG before there were regular roads, and were and are being used in places with no roads at all. So it's not roads alone that are the answer.

Very probably, in this experiment, the pigeons were taken from their homes to their launch points by vehicles using roads. Even if the birds couldn't see the highways, they probably (on an instictive level if nothing more) figured out that their outward trip involved roads, and once aloft they used the nearest road to find their way back.

There is also the possibility that roads, being clear of grass and trees and mountains, provide different air movement than natural landscape and the birds follow the more regular air flow that would be above roads because it's easier than flying over natural (and uneven) landscapes.