As an alternate to IWB, I gave ankle carry a try on a recent road trip to Indiana (about a 10 hour drive for me). Paired with a sufficiently light fun, it made for a very comfortable way to carry while still being accessible. To me, it was easier than trying to wrestle with my seat belt, especially if the gun was also covered by a shirt and jacket. A simple lean forward along with pulling up my pants leg gave quick and easy access. Of course, with a larger gun, this may not be a valid option.
Another option that Darryl Bolke recently wrote about online was a shoulder rig for extended times in the driver's seat.
If IWB works for you, then that's a great option, especially if you're in and out of your car.