The usual way of addressing this issue ?" sometimes called "testing by exhaustion" ?" involves testing these systems until you're satisfied they're safe. But you can never be sure that this process will uncover all potential flaws. "People carry out tests until they've exhausted their resources and patience," said Sayan Mitra, a computer scientist at the University of Illinois, Urbana-Champaign. Testing alone, however, cannot provide guarantees. Mitra and his colleagues can. His team has managed to prove the safety of lane-tracking capabilities for cars and landing systems for autonomous aircraft. Their strategy is now being used to help land drones on aircraft carriers, and Boeing plans to test it on an experimental aircraft this year. "Their method of providing end-to-end safety guarantees is very important," said Corina Pasareanu, a research scientist at Carnegie Mellon University and NASA's Ames Research Center.