Sure, the way I'm imagining it is some sort of collision avoidance system, where the person drives but the computer can also put on the brake? It seems like that could be safer (although it could still cause an accident by braking in certain situations like in an intersection). If the self-driving system does other things to reduce accidents like leaving greater margins of safety for other drivers (by stopping less suddenly, pulling out into intersections differently, etc), it seems like that would be harder to integrate with a human driving, instead of having the self driving system take over.
A lot of the utility of self driving cars is not just increased safety. It's allowing disabled people to be more independent, reducing the need for car ownership with (less expensive) self-driving taxis, reducing the need for parking lots and allowing that land to be used differently, etc. Do you think self driving cars should be prohibited as long as they are less safe than the standard set by human+collision avoidance cars?
I don't think that makes sense unless cars with human drivers and no collision avoidance are also prohibited. Even then I'm not sure. As a society we know that driving is not totally safe but allow people to drive anyway because it's a useful tool. I don't see why the safety standard should be even higher for self-driving cars, which are even more useful, especially for people who can't drive.
One last thing is that anything that slows the adoption of self-driving cars will probably lead to a much greater number of old, less-safe, cars in use for a longer period of time, since it's very unlikely politically that the existing car stock will be just kicked off the road, regardless of what regulations are set for new ones.