Washington State Ferries is expecting nearly a half million riders on state ferries through the Memorial Day holiday.

The crush of holiday travelers prompted late ferry sailings Thursday, but delayed boats on the Bainbridge Island-Seattle route early May 23 were caused by another culprit: foggy weather.

Morning sailings of the M/V Walla Walla between Seattle and Bainbridge were set back an estimated 18 minutes behind schedule due to fog and reduced visibility, according to WSF.

The Walla Walla got back on schedule before noon Thursday.

It didn’t last, however, as sailings of the Walla Walla slipped to an estimated 25 minutes behind schedule by the early afternoon.

The boat, as well as the M/V Wenatchee, were both running late on the Bainbridge route until after 9:30 p.m. Thursday.

WSF is advising ferry travelers, especially drivers, that they should be ready for lengthy waits.

The busiest travel times for ferries will be through Saturday westbound to Kitsap Peninsula and the islands, then Sunday through Tuesday for travelers eastbound back to the mainland.

Also, WSF said that travel around Colman Dock in Seattle will be particularly busy due to several nearby construction projects.