The pilot program to offer reservations for parking spots in Yosemite Valley shows what can happen when visitor numbers double and the infrastructure stays the same.

By 11 a.m. most summer days in Yosemite Valley, there’s no place left to park.

Sound familiar? The same thing often happens on weekends at many Bay Area parks. A parking crunch. It also can hit on weekdays at parking lots for BART, the ferries and at Peninsula Caltrain stations. Everything is full.

Over the past 20 years, the Bay Area population has more than doubled from 3.5 million to 7.6 million (including another 90,000 in the past year). Except in a handful of cases, roughly 250 parks have parking lots that are the same size as in 1995. On Saturdays and Sundays, they fill quickly.

In Yosemite National Park, visitation has increased from 984,000 in 1955 to 2.5 million in 1975 and 3.9 million in 1995. In the past three years, visitation has been 3.9 million, 3.95 million and 3.85 million, respectively.

One plan to alleviate crowding has been the YARTS bus, which provides service to Yosemite from several gateway towns outside the park, such as Mariposa. In 2014, more than 1 million riders took YARTS to Yosemite. But with nearly 4 million visitors, searching for a parking space still can feel like looking for a polar bear in the desert.

The pilot program allowing visitors to reserve parking spaces will take place on the weekends of June 25-26 and July 2-4. Parking reservations will cost $1.50 and be available at www.recreation.gov, the same service that sells out of reservations for campgrounds at Yosemite so quickly when they become available.

The park will designate 150 parking spaces near Yosemite Lodge. That site is within short walking distance of the trail to the base of Yosemite Falls, as well as a station for the park’s shuttle bus.

In the Bay Area, the only sure-thing answer to the parking crunch has been to avoid peak times and visit parks on early mornings or Mondays through Thursdays.

The basic problem is unsolved: Population growth needs to stop or infrastructure needs to match its growth.

Tom Stienstra is The San Francisco Chronicle’s outdoors writer. Email: tstienstra@sfchronicle.com Twitter: StienstraTom