I have found that it is more a matter of ease and speed to cut the belt rather than try to reach in and unbuckle the belt. In pulling casualties out of vehicle hit by roadside IEDs, it was always quicker and easier to cut and pull them out. There were always straps and equipment hanging up on something when you are trying to pull them out, and if you are under fire at the time, or the vehicle is burning, climbing in is not a good idea. I almost always used a pair of EMT shears for the job. They are safe and quick. Not your typical automobile crash, and anecdotal.

I keep two pairs of EMT shears in my vehicle, one on each side of the front. This way either person can reach them and we can reach into the back seat. Not expecting any roadside bombs here, but some habits I have not been able to break yet. I have used them often though; mostly for opening things and simple cutting jobs. I just prefer shears to a knife in an enclosed space.