If one is ever developed that works well it will be very expensive, as is almost any machine that is trying to do two very different tasks because of the design compromises that have to be made. A Cessna 182 is mechanically a much simpler machine than a Chevrolet car but costs many times more not only to buy but to own because of the scheduled maintenance and inspection costs.
Getting back to the original question there is no doubt in my mind that autonomous-driving cars will be safer than humans and should be implemented ASAP. I spend 18 years on the road as a police officer and wrote what I thought was a pretty impressive stack of accident reports, and I didn't even work in the traffic section.
NOT ONCE did I see a collision that could not have been avoided by reasonably proficient drivers who were sober, driving within the traffic laws, and paying attention to the task of driving their own respective cars.
Frankly I can't wait for self-driving cars and firmly believe that overall the world will be a much safer, if not as exciting, a place. For those who feel that being in control of their own car is a constitutional right, I hate to bust your bubble but every state maintains that driving is a privilege, not a right. I believe that most other countries do too. (If not someone will correct me I'm sure.
) Once self-driving cars are proven and affordable those who insist on driving themselves will be welcome to do so at the local track and will not be allowed to risk other people's lives to prove that their outdated skills can beat a computer.
Consider the parallel with vaccines, which have in many cases have virtually eliminated once-deadly and crippling diseases, at least in third-world countries. But once in a great long while, some unfortunate person has a bad reaction to a vaccine and may be seriously injured or killed by it. I doubt anyone would argue that society should give up vaccines though.
Oh, and computers have already put the jobs of a lot of fighter pilots at risk:
This AI Can Beat a Top Fighter Pilot - D-brief
If a computer can outfly a fighter pilot I'm sure one will be along that can drive a car or truck. It's only a matter of time.