The draws of the Sacramento County highway bridges, mile 4.5 near Isleton, and mile 12.4 near Walnut Grove, shall open on signal from 6 a.m. to 10 p.m. from May 1 through October 31. The draws shall open on signal from November 1 through April 30 from 9 a.m. to 5 p.m. At all other times, the draws of these bridges shall open on signal if at least four hours notice is given to the drawtender at the Rio Vista bridge across the Sacramento River, mile 12.8.