Rather than the "Thigh Rig" I prefer a cross draw or shoulder holster when driving long distances. The Serpa when mounted on the pancake adapter can be angled for comfortable cross draw. This places the firearm outside the interference of the seat belt. In my vehicle there is a large center console that would be somewhat of an obstacle when drawing from strong side. It would seem to me that drawing from a thigh holster while seated in the vehicle could be limited by your elbow hitting the seat back. Another plus for cross draw is that the firearm is somewhat hidden from anyone outside. Drivers side it's below the side window. Passenger side it's hidden by your body. In the professional Bodyguard/Driver business, this is often the manner of carry. Good easy access and comfortable for long periods seated behind the wheel.
Whatever you do, make sure that the firearm is SECURE. One of the deadly mistakes in the famous (at least notorious) FBI shootout in FL was that an agent placed his main firearm on the seat during a chase. When he wrecked, the firearm "disappeared" somewhere on the floor or under the dash. When he took immediate fire, he was stuck with a much smaller, and less effective, backup weapon.
Like maclean said, drive or fight. If your driving that weapon MUST be secured. On your body is best as you won't have to go looking for it should you have to leave the car. If you must put it somewhere in the vehicle a good holster, secured to the seat or console would be next best.
