A 2016 study by NREL identified 8,130 million square meters of suitable rooftop area for PV installations in the continental U.S, amounting to 2,008,966 acres, which could potentially supply up to 14.8% of the entire U.S. electric power consumption.
According to the UC Berkeley's Department of Civil and Environmental Engineering, there are about 3.4 parking spaces in the USA for each car, and many well-cited and informed guesstimates say there may be as many as 8 outdoor parking spaces per car. With about 250,000,000 cars in the US, and the assumption that only 2 parking spaces with 180 sqft per car would be suitable for solar installations, one arrives at a photovoltaic parking area potential of ~ 2,066,115 acres.
Using different data sources, one arrives at similar metrics for parking-space solar potential. As of September 2020, there were 213,300,000 single-family dwellings in the United States and 38,000,000 multifamily units. If 2 outdoor parking spaces with 180 sqft per housing unit were covered with solar panels, one could cover approximately 2,076,859 acres with solar panels.
Both estimation methods would arrive at slightly larger solar potential than the solar rooftop potential determined by NREL in 2016.
Without counting any commercial or institutional parking areas, the above numbers lead to the conclusion that driveways and parking spaces offer an underexploited solar generation potential, combined with an underserved market potential of ~250,000,000 suitable private residences x $18000 = $ 4,500,000,000,000.