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.