This is the harm-reduction debate. For example, Swedish snus and electronic cigarettes are much less harmful than smoking, but the argument goes that quitting nicotine altogether is less harmful than anything. Yet people still smoke and use nicotine, so switching the stalwarts over to less-harmful methods of nicotine consumption would save lots of lives and money.

Likewise, it may not be possible in the short term to convince everyone to stop driving cars. Self-driving cars won't fix the long-term problem of efficient transportation, urban sprawl, or emissions, but they can reduce its harm by efficiently providing car-like service to the public.