Self-driving cars, ride sharing, and other mobility services should slash sales of sedans by nearly 40% by 2030, as Americans become less reliant on vehicle ownership, according to a forecast from KPMG.

Sedan sales are expected to fall to 2.1 million units by 2030, a 38% decline from the 5.4 million level of sales today, the firm predicted.

Autonomous mobility services will transform the nation's transportation market from a national to regional one with 169 "islands of autonomy" — urban areas with their own transportation and consumer travel needs.

"Across the world, a $1 trillion market is swiftly developing around a new and disruptive transportation mode: driverless vehicles coupled with mobility services," said Gary Silberg, KPMG's automotive sector leader. "However, the adoption of this new transportation mode will not be immediate, and it will not be everywhere. Instead, it will arrive in what we call 'islands of autonomy.' Each island will need a unique mix of vehicles to meet unique demands, which will greatly impact the breakdown of the car park, especially sedans."

The firm predicts the "demise of the personally owned sedan" will cause several automakers to cease production of vehicles in the segment.

"Several OEMs will likely close plants and exit the segment entirely," said Tom Mayor, KPMG's strategy leader for industrial manufacturing. "At these volumes, we would expect the current 10 OEMs serving the U.S. market with more than 800,000 sedans per year to contract to only three or four."

To be profitable in this market, vehicle manufacturers will need to develop a "trip-by-trip understanding of consumer needs in each island, aggregating across the islands to find scale and then developing new classes of vehicles specifically designed to optimally meet the varied trip-level needs of consumers who will hail for most trips rather than own or lease," Mayor said.