For the traffic light problem, I'd start off by programming the car with common scenarios, and when you hit one it doesn't understand, throw up a prompt that tells the driver to tap the accelerator when it's ready to go. You can then improve from there. "Driverless" doesn't have to be all or nothing. We can ease into it.

For the plastic bag, I find it odd how often people bring up scenarios like this that will supposedly stump a driverless car, yet humans are terrible at as well. "What if a massive truck comes careening down the road straight at you, and your only avenue of escape is through a playground full of children? What does the car do?" Well, somebody dies, just like now. My experience is that humans are terrible at deciding whether some crap in the road is safe to run over, so computers can hardly do worse. And I think they could easily do better. A front-facing radar (already standard equipment on many cars) will tell you whether that lump the camera sees is actually substantial or just filled with air.