Well, it would have to be more dangerous than what it's competing with -- that being, human-piloted automobiles. I'd say, one fatality per month would be a great success, except that doesn't figure in the control-freak nature of American culture. So what if I'm more likely to die in a rolling car than in a flying car? At least I'm in control of my rolling car!
Of course, there's a huge industry around cars, and a huge driving culture in this country. I think many car companies, and their well-placed politicians, will do whatever they can to keep the status quo going.
Yeah...I think one of the strongest arguments for evidence of national corporate conspiracies involves GM et al working to curtail electric public transportation as much as corporately (I was going to say humanly) possible...
Variation on self driving cars...public transportation! I'm discovering that I miss my London twice a day 25 minute reading sessions (even standing up sometimes.