Sometimes visiting a national park is a spontaneous decision that’s part of a last-minute weekend excursion or impulsive road trip. Now, Maine’s Acadia National Park is making that spontaneity a bit more difficult by testing a timed-entry reservation system this fall, for select popular spots around the park. If all goes well, the full system will be implemented in 2021, from the second Friday of June to Columbus Day.

Acadia will be following the example of Muir Woods and Haleakala National Park, which have similar reservation-based rules. The permit system is intended to prevent gridlock traffic, spread out visitors more evenly across different days, and reduce the amount of unsafe road parking during peak season, particularly around Cadillac Mountain, Ocean Drive, and Jordan Pond.

Acadia, which was the seventh-most visited national park in the country in 2018, sees around 3.5 million annual visitors, but it wasn’t exactly built to accommodate that many people.

The permit system will give visitors a specific window of time, between 7:00 AM and 5:00 PM, to enter certain designated areas, though they will be allowed to remain there for as long as they wish. For Cadillac Mountain, the timed-entry reservation will start much earlier and end much later because visitors want to be at the summit for sunrise and sunset.

When it is fully implemented, the system will only apply to certain busy sections of the park during the summer months, and may cost an additional $10.