After years of anticipation, autonomous vehicles are now being tested on public roads around the world. As ambitious innovators race to develop what they see as the next high-tech pot of gold, some experts warn there are still daunting challenges ahead, including how to train artificial intelligence to be better than humans at making life-and-death decisions. How do self-driving cars work? How close are we to large-scale deployment of them? And will we ever be able to trust AI with our lives?