Vehicles that move through difficult terrain are not slowed down like infantry – they will either crash through successfully or become stuck, bogged down or something similar. To represent this, a vehicle that moves into difficult terrain or starts its move in difficult terrain must take a Difficult Terrain test. Do this by rolling a D6. On a roll of 1, the vehicle is immobilised for the rest of the game, on a 2-6 it can carry on moving. The test is taken when the vehicle first encounters difficult ground during its move and will halt the vehicle there if it fails the test. If moving through terrain deemed especially hazardous, then vehicles are immobilised on a roll of 1-3, rather than merely a 1.
Designer's Note: This rule has been changed slightly to clarify where in its move a vehicle is halted – ideally it would be at a random point in the move, but this generates far too much complexity. As the idea is to deter vehicles from entering difficult terrain and sticking to the open spaces, having them crash on encountering obstacles is the easiest abstraction. Also, the option to define terrain as being particularly hazardous to vehicles has been included in order to curb excessively dangerous driving.
There are many different types of terrain. The following table shows some of the more common types and explains whether they count as difficult ground to vehicles or not.
| Terrain | Effect |
|---|---|
| Sand or ash dunes | Difficult ground |
| Marsh, muddy or water-logged ground | Difficult ground |
| Scrub, brush, vines or thick bracken, dense undergrowth | Difficult ground, except to tracked tanks which are heavy enough to flatten it as they move, and other vehicles moving 6" or less. |
| Steep hillsides | Difficult ground. Sheer rock faces are impassable to vehicles. |
| Dense woodland, jungle | Difficult ground |
| Rubble, ruins, detritus, slag heaps, crumbling rock | Difficult ground except to tracked tanks which are moving 6" or less. |
| Buildings | Impassable to all but tracked tanks. Even they treat buildings as difficult ground and if they move more than 6" must treat it as especially hazardous (see Difficult Terrain). Some large buildings may be completely impassable to vehicles no matter how recklessly they are driven. |
Designer's Note: These categories have been included to give some idea of how different terrain types affect vehicles. It would be a bad thing to curb creativity by defining exact terrain types, so it is particularly important for players to agree effects before commencing a game. The terrain generators in the Warhammer 40,000 rulebook include many ideas for terrain and provide a good starting point.