So, you're asking if my pistol is in its holster, as opposed to ....where exactly?
On the car seat? Console? Under my thigh? Wedged between cushions?
In any of those cases, a collision can send the weapon bouncing around inside the car with undesirable results.
Inside the console or glovebox? Why make it harder to access?
If your firearm's carry position puts the gun out of reach while sitting and driving, perhaps it's time to consider changing your carry location. Or obtaining a snap-on holster that allows you access while driving.
I've found that on a long road trip, a good shoulder holster is the most comfortable thing for me. Topped with a light nylon windbreaker in summer it makes getting in and out of the car easier and nothing digs against me. Alternatively, a cross-draw holster canted way over works well for others.