Practice...
if you want to get better.
First, as others have sagely pointed out, your best move in a hypothetical that you're thinking about is to leave. In order to do that more effectively, don't tailgate. When stopped leave enough room so you can get out if need be.
Now, to answer your question, let's assume for the sake of discussion that you've done this, but some butt-breath in an old beat up Ford Exploder decides 'hey, that huge gap looks nice, so I'll take it'... and he eats up all your extra space. Traffic is ground to a halt and you're the only one around in a Lexus. Wouldn't you know that your local crack-fiend sees your ride and being a firm believer in the democratic party platform of wealth redistribution, he decides to rob from the rich, and give to the poor - himself. Out comes an object in his hand, you cannot tell what, but let's say for the sake of discussion it's very shiny. Now what? <if you do practice for this, please practice without a loaded gun>
gun on the belt:
When you practice a typical 5 step draw, step one should bring your shooting hand to the butt of the gun, brushing concealment out of the way positively, and support hand up in front of your abdomen to smooth the meet/grip after the gun clears the holster. Sitting in a car or truck, you won't be able to do this as easily with the belt on, so this has to be modified to undo the belt and clear any concealment garments
LH - with right hand, go to the seat belt release and click it off, with left hand, clear away the concealement (if any) and get to the butt of the gun, ready your support hand. From this position you should be able to present fairly quickly.
RH - it will be more clumsy; Click off the belt with the right hand, if need be, clear it with the left. Move your right hand under any cover garments and to the grip of the gun, and move your support hand to a position ready to 'grip'.
Here's a tip - chances are, the steering wheel is going to find it's way into your natural arc of movement to accomplish a draw. Slow, smooth and sure is better than getting tangled up in your seat belt then hitting your steering wheel in the draw...