The sensors in an autonomous cars are what are used to let the system understand relative distances. But this becomes difficult when there are adverse weather conditions, or signs with graffiti.
The technology used by self-driving cars is called machine learning. It helps the machine make decisions, and patterns the algorithms. But there is some doubt about its safety at the moment.
patterns of learning
Autonomous cars are continuously learning, once they get on the road. However, there is no way of knowing what patterns of safe behavior the car will learn, and whether there are ways to control that.
As of now, there are no standardized rules and regulations to guide the use of autonomous cars. Moreover, there are no international safety standards that the cars have to meet.
The number of accidents involving autonomous cars is alarmingly high. Therefore, getting them on the road doesn't just involve acceptance from the people within the car, but also that of those who will have to drive alongside.