[Imp]

Just brushing over but read the game guide 2 things effect your chance of getting stuck, the vehicle class & speed.

Vehicle classes
2WD Truck & Utility - Very poor avoid sticky terrain
4WD Truck & Utility - Nearly as good as track
Track - very good you can treat most multi wheel APCs as this class for chance of sticking.

Speed
The faster you go the higher the probabilty you will get stuck in a sticky hex so entering it as the first hex moved is the least chance getting worse the faster (further) you move

So glossing over most common sticky terrain

The Worst
Mud & Swamp even 1 hex move has a risk of getting stuck, very rare I drive on & 2nd hex is very high risk.
Avoid unless really really have to & only ever enter as the first hex moved into

Best
Rough inc rough trees. Can happily drive over within 4 hexes in fact can push further as follows.
Safe hexes = Vehicles total MP divided by 4 so 20MP 5 safe 24MP 6 safe hex etc
Rember firing or loading unloading uses MP take them into account.
By safe I mean ZERO chance of getting stuck or so low I would call it 99.9%
Note Rough slopes are worse & should be avoided or only entered as the first hex of the move, they have a slight chance of getting stuck even then.

Fords nearly as safe would not try over 4 hexes & 4WD like a jeep a bit closer

The rest
Entering as first hex entered is pretty safe or safe, 2nd hex needs a reason you are starting to gamble. Each additional hex increases your chance of losing the bet.
Slopes & any combo are considerably worse than flat so avoid or first hex only but still risking