Motorways and freeways are the low-hanging fruit of autonomous driving; everyone is moving in one direction at the same relative speed, and there are no pesky pedestrians to get in the way. Much of what is passed off today as “autonomous driving” is some variation of this sort of advanced cruise control.

But there is an elephant in the cab with even this rudimentary form of autonomy. Many companies are planning cars that, in the event of an emergency, hand back control to the human driver. (Google, a notable exception, plans a car with no steering wheel or brake pedal.) The potentially fatal weakness of this strategy is that it assumes “drivers” will be paying attention at the split second they are most needed, instead of being busy, say, taking a nap.

The much harder, and still mainly unsolved, autonomous driving problem involves not highways but cities, with all their chaos and complexity. Self-driving cars still struggle with simple potholes; no one has come even close to demonstrating a completely driverless car that could do the work of a Manhattan taxi driver on a rainy day.

The sad reality of autonomous car technology is that the easy parts of have yet to be proven safe, and the hard parts have yet to be proven possible. We’re nowhere close to Silicon Valley’s automotive “Tomorrowland.”

The most realistic industry projection about the arrival of autonomous driving comes from the company that’s done the most to make it possible. Google, while never explicitly saying so, has long intimated that self-driving cars would be available by the end of the decade.

In February, though, a Google car caused its first accident; a bus collision with no injuries. A few weeks later, Google made a significant, if little-noted, schedule adjustment. Chris Urmson, the project director, said in a presentation that the fully featured, truly go-anywhere self-driving car that Google has promised might not be available for 30 years, though other much less capable models might arrive sooner.

Historians of technology know that “in 30 years” often ends up being “never.” Even if that’s not the case here, if you’re expecting a self-driving car, you should also expect a wait. And so you might want to do something to pass the time. Maybe go for a nice drive?