A year ago, Detroit and Silicon Valley had visions of putting thousands of self-driving taxis on the road in 2019, ushering in an age of driverless cars.
Most of those cars have yet to arrive — and it is likely to be years before they do. Several carmakers and technology companies have concluded that making autonomous vehicles is going to be harder, slower and costlier than they thought.
“We overestimated the arrival of autonomous vehicles,” Ford’s chief executive, Jim Hackett, said at the Detroit Economic Club in April.
In the most recent sign of the scramble to regroup, Ford and Volkswagen said Friday that they were teaming up to tackle the self-driving challenge.
He and others attribute the delay to something as obvious as it is stubborn: human behavior.
Mr. Salesky said Argo and many competitors had developed about 80 percent of the technology needed to put self-driving cars into routine use — the radar, cameras and other sensors that can identify objects far down roads and highways. But the remaining 20 percent, including developing software that can reliably anticipate what other drivers, pedestrians and cyclists are going to do, will be much more difficult, he said.
A year ago, many industry executives exuded much greater certainty. They thought that their engineers had solved the most vexing technical problems and promised that self-driving cars would be shuttling people around town in at least several cities by sometime this year.
Companies like Waymo and G.M. now say they still expect to roll out thousands of self-driving cars — but they are much more reluctant to say when that will happen.
Waymo operates a fleet of 600 test vehicles — the same number it had on the road a year ago. A portion of them are the first set of vehicles it will be buying through the agreements with Chrysler and Jaguar. The company said it expected to increase purchases as it expanded its ride service.
Tesla and its chief executive, Elon Musk, are nearly alone in predicting widespread use of self-driving cars within the next year. In April, Mr. Musk said Tesla would have as many as a million autonomous “robo taxis” by the end of 2020.
Tesla believes its new self-driving system, based on a computer chip it designed, and the data it gathers from Tesla cars now on the road will enable the company to start offering fully autonomous driving next year.
“If you’re out driving 20 hours a day, you have a lot of opportunities to see these things,” Mr. Salesky said.
The technology is available now to create a car that won’t hit anything. But such a car would constantly slam on the brakes.
“If the car is overly cautious, this becomes a nuisance,” said Huei Peng, director of Mcity, an autonomous-vehicle research center at the University of Michigan.
Also this year, a Boston start-up, Optimus Ride, plans to begin operating driverless shuttles at the Brooklyn Navy Yard that also travel at 25 m.p.h. or less.
Ms. Malek said she believed it would take years and perhaps even a decade or more to develop driverless cars that could travel anywhere, any time.
“Our focus is, how can we use the technology today?” she said. “We realize that today we have to start somewhere.”