One of the biggest challenges in building autonomous cars is the safety issue which need to be resolved completely before a single autonomous car hits the road. Events such as DARPA grand challenge are designed to improve the capabilities of such vehicles.
A recent story in cnet.com talks about designing safe autonomous cars, which have collision avoidance capabilities incorporated into them. The inspiration came from the mother nature again, this time the movement basics of a large group of fish. Visit Cnet website for more.