I would consider it 99% complete when I can pull up a map, point to any stretch of drivable road in the United States and ask it to autonomously operate there.
Specifically, there's an especially tricky road in I-35N in Texas where the lane markings come ago, yellow shoulder markings _merge_ into barriers, and the road condition is so bad that the steering rack will _turn_ by itself(!). We're not even close to calling this thing autonomous. If and when it gets to that point, I would consider it "99%" complete.
In reality though, I really see this tech being viable in continued controlled conditions. Maybe there will be a point when certain lanes will become dedicated "autonomous lanes", where they're completely isolated from the rest of traffic, come with special markings and sensor suites to assist driveler-less cars operate efficiently.
As long as the chaotic human element is ever present, regardless of how well they model these systems, it will fail to cope with the massive amount of discontinuous information humans inject into a situation (swerving into a lane, road rage, cargo suddenly coming loose from a truck, etc.)
Oh, and have we tested these things during winter? :p