Which lot?
If you are going to Lot 1 (near Hammer' s Lot and Pinto Ken's big tailgate at Lot1 Pole 5):
Take 20 down to 240/277,. Take that to "downtown" OP and turn right onto 20A (Big Tree) . Keep going past the highway (219) and the entrance to Lot1 will be on the right.
If Southwestern is backed up to the big curve (where it changes from Transit to Southwestern) you can take the backroads straight to 20 A
The route you gave takes you to the north side of the stadium and and from the messages on this thread, the road work is between California Rd and the Stadium on 20 (Southwestern).