I'm surprised nobody offers 5 point harnesses as in a serious accident, I'd rather have that than the single lap/shoulder belt. Strap me in like NASCAR!!!!
That would be very uncomfortable, and exceptionally unsafe, in a street car.
For one, a harness, properly secured, means your torso can't move, PERIOD. Forget leaning over a bit to clear a blind spot in the A pillar, let alone looking over your shoulder for traffic.
Two, check out the seat in a NASCAR car, or any other serious race car. You'll never find a plush, comfortable seat like you will in a street car. Plush seats means your body can fall away from the harness, and that's BAD in an accident, as the impact forces will throw your body into the harness causing all sorts of injuries by itself. There have been cases of spinal compression injuries from people running harnesses on top of plush seats (my personal opinion - putting harnesses on any kind of suspension seat is just begging to get paralyzed one day).
Three, watch videos of those guys getting strapped into the cars. Often times there will be a 2nd man outside the vehicle that will pull the straps tighter than the driver can get them. Properly adjusted, those harnesses are TIGHT on the driver, ensuring that the driver's body can't move around. Non properly adjusted harnesses CAUSE injury in an accident, not prevent it.