Some Interstates yes, for example the 7.5 miles between Las Pulgas and Oceanside on the CA 5 Freeway. Sure you can ride through the base if you show your I.D. but some still prefer the freeway shoulder (plenty wide enough, but sometimes dead-tire litter, etc.).

Also, on the Joshua Tree Double Century, the route included almost 40 mi. on the CA 10 freeway because there was no alternative route. All legal and again, a plenty wide shoulder on the freeway.

Rick / OCRR