Roads are a terrain element in the game R.U.S.E. which affects the behavior of units, like Woods and Cities. Roads provide no defensive or offensive abilities, but are nonetheless crucial to the effective deployment and routing of your troops. Units that travel by Road move at normal speed, which is the speed listed on their entry, either here on the Wiki or on the in-game 'Rusopedia'.
When certain units (like Infantry) are forced to travel off-road, their speed is halved, making troop deployment a lengthy process. The effects of a Blitz plan counteracts the slowing effect of rough terrain, and doubles a unit's base speed when traveling by road.
Tracked vehicles, like Tanks, are not susceptible to this penalty. Their speed remains constant whether off-road or on, but still gain the double speed benefits of a Blitz Plan.
Engineering trucks and supply trains always travel by road. The only time an Engineering Truck will turn off the road is to deploy a Building. Also roads almost always cut through Cities at some point or another, which makes them tempting locations for enemy ambushes. Bear this in mind when routing your troops.