DETROIT (Bloomberg) -- U.S. auto sales may drop about 40 percent in the next 25 years because of shared driverless cars, forcing mass-market producers such as General Motors and Ford Motor Co. to slash output, a Barclays Plc analyst said.

Vehicle ownership rates may fall by almost half as families move to having just one car, according to a report published today by the analyst, Brian Johnson. Driverless cars will travel twice as many miles as current autos because they will transport each family member during the day, he wrote.

Large-volume automakers “would need to shrink dramatically to survive,” Johnson wrote. “GM and Ford would need to reduce North American production by up to 68 percent and 58 percent, respectively.”

Self-driving cars have become a frequent topic for auto executives as the technology for the vehicles emerges. The market for autonomous technology will grow to $42 billion by 2025 and self-driving cars may account for a quarter of global auto sales by 2035, according to Boston Consulting Group. By 2017, partially autonomous vehicles will become available in “large numbers,” the firm said in a report in April.

Johnson’s report, entitled “Disruptive Mobility,” contends that the shift to cars that drive themselves will upend the auto industry.

GM and Ford didn’t immediately respond to requests to comment on Johnson’s report.

When most vehicles are driverless, annual U.S. auto sales will fall about 40 percent to 9.5 million, while the number of cars on American roads declines by 60 percent to fewer than 100 million, he estimated.

'Historical precedent'

“While extreme, a historical precedent exists,” Johnson wrote. “Horses once filled the many roles that cars fill today, but as the automobile came along, the population of horses dropped sharply.”

Automakers are working to overhaul their business models for a world where mobility is being redefined as most of the global population crowds into large megacities during the next two decades. Driverless cars that move in harmony may become essential to keep people and goods flowing safely and efficiently.

Johnson foresees four vehicle categories -- traditional cars and trucks driven by individuals for work or in rural areas; “family autonomous vehicles,” owned by individuals and shared by a single family; “shared autonomous vehicles” that would be “robot taxis” summoned by smartphone; and “pooled shared autonomous vehicles” that accommodate multiple riders, like a bus or a van.

Every shared vehicle on the road would displace nine traditional autos, and each pooled shared vehicle would take the place of as many as 18, according to Johnson’s report.

Mobility savings

Consumers’ cost of mobility would drop dramatically, he wrote.

“By removing the driver from the equation (the largest cost in a taxi ride), the average cost per mile to the consumer could be 44 cents for a private ride in a standard sedan and 8 cents for a shared ride in a two-seater,” Johnson wrote, noting that would be “well below” the $3 to $3.50 a mile consumers now pay to ride in an UberX car or the $1 to $1.50 a mile for an UberPool vehicle.

The shift to driverless vehicles would benefit transportation network companies such as Uber Technologies Inc., autonomous technology providers such as Mobileye NV and makers of low-cost vehicles, according to the report.

“While the Detroit Three will still have pickups and vans (their most profitable segments), they will be challenged in” the markets for family autonomous vehicles and shared vehicles, Johnson wrote. “We see this as a further lid on the prospects for” traditional, mass-market automakers.