Vehicles, other than skimmers or walkers, who follow a road for the entire Movement phase may add 6" to their maximum speed. They may only do this if their move required no Difficult Terrain tests, if they do not shoot, if they do not embark or disembark passengers and they do not perform a Tank Shock.
Roads
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 37