SAN FRANCISCO — In the minds of many in Silicon Valley and in the auto industry, it is inevitable that cars will eventually drive themselves. It is simply a matter of how long it will take for the technology to be reliably safe.

But as indicated by Google’s challenges with the so-called handoff between machines and humans — not to mention Uber’s problems during recent tests on the streets of San Francisco — there is a lot more work to be done before self-driving cars are ready for the mainstream. Here are some of the challenges facing technologists.

The ability to respond to spoken commands or hand signals from law enforcement or highway safety employees.

There are subtle signals that humans take for granted: the body language of a traffic control officer, for example, or a bicyclist trying to make eye contact. How do you teach a computer human intuition? Perhaps the only way is endless hours of road testing, so that machines can learn the interactions that humans have been socialized to understand.