Well, with the reverse dice pool it seems simple enough.
Driving on a highway: Difficulty 2
Minimum driver competence: Wits 1 Drive 1
Dice rolled: 0
Bad weather: Difficulty +1
Reducing travel time by raising Difficulty: +1
So now you're speeding on a highway in the rain. You have two dice worth of failure.
If instead you intentionally go slowly to adjust for the weather, you might lower difficulty.
The tricky bit comes in that you have to bet your pool before you roll, the Gm might reveal hidden modifiers to the base situation(like your brakes having been sabotaged).
Basically if theres nothing at stake you might as well take the difficulty reducers, check that you indeed do have more bonus than pool, and say so. It works automatically.