That is the one real reason to ever consider one IMO. A horizontal shoulder rig is even better IMO (i had a Galco Miami classic that i used for many years with my S&W 9mm), though i don't wear a seatbelt. With a seatbelt on drawing from one might be problematic.
Under the thigh is fast, and provided you have the right kind of gun is reasonably safe, though if you were in an accident im sure it would go flying down onto the floorboard, which could obviously be problematic if someone just ran you off the road in a fit of rage, or something like that.
I have seen specialized car cross-draw holsters before, they are around.
I just got a glove-like synthetic Blade-tech IWB holster for my P7, that's what i most prefer for real world day in and day out carry. I'll take a pic and post it for ya.
Here ya go:
I wear both these holsters in the same area, behind my strong side hip. Both have the 15 degree "FBI cant," and can be drawn from from a strapped in seated position, though it's not exactly fast.
Synthetic setup is IWB Blade tech and Fobus paddle double pouch.
Leather setup is a Galco belt slide and clip on don hume single mag pouch.
I prefer no thumb break on my holsters.
I had a couple custom holsters made for my kel tec p32 but to be honest i prefer to just carry it with the integral belt clip.
DRIVING HOLSTERS:
driving holster - Google Search