On the flank of Mount Tamalpais, all the trailhead parking lots — Bootjack, Pantoll and Rock Springs — often are filled by mid-morning on weekends.

Some 350 miles north, a mind-bending drive for most, all the campsites are filled this week at Jedediah Smith Redwoods State Park.

At Lake Tahoe, rangers have had to post a billboard-type sign to block the entrance of D.L. Bliss State Park because no parking spaces are left. In Yosemite Valley, all the parking spaces are filled by 9 a.m. and cars parade in a continuous loop for a site.

With July 4 falling at midweek, many people are adding vacation days to their weekends and have turned this week into a vacation logjam, near and far. This is not the week to head off on a seat-of-your-pants trip without reservations.

The heart of the problem is that the population of California has doubled since 1970, from roughly 20 million to 40 million, while the number of campground spaces at state and national parks has remained roughly the same.

In the Bay Area’s nine-county region, the population has more than doubled in the past 25 years from 3.5 million to 7.8 million. Except in a handful of cases, roughly 250 parks have parking lots that are the same size they were in 1995.

At Yosemite, visitation has increased from 984,000 in 1955 to 3.9 million in 1995 and to more than 5 million in 2016. The number of campsites and lodging rooms available has stayed about the same.

From the viewpoint of a traveler, from July 4 through the start of school in August, you must have reservations for trips to well-known destinations. Going Tuesdays or Wednesdays can still provide openings and opportunities.

A good strategy is to save your vacation time for post-Labor Day Weekend. On Tuesdays and Wednesdays, many parks, campgrounds, lakes and trailheads turn into open playing fields.

From the standpoint of rangers, at Muir Woods National Monument, they are trying reservations for parking spaces. The same strategy has worked during the waterfall season on weekends at Uvas Canyon County Park near Morgan Hill, and was tested without much of an impact in Yosemite Valley.

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