This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
Whether bicycles can ride on an Interstate varies segment-by-segment in California. Cal. Veh. Code § 21960 authorises Caltrans and local authorities to prohibit pedestrians, bicycles, and motor-driven cycles on any freeway or expressway by sign. In practice, every urban Interstate segment in California is signed prohibited; many rural Interstate shoulders (notably parts of I-5, I-8, I-10, I-15, I-40, and I-80) are signed open. Always check the Caltrans bicycle-on-freeways map for the specific corridor before riding.
What this means in practice
Check the California DOT's bicycle-on-freeways map before you ride. Watch for "no bicycles" signs at on-ramps — they're posted segment-by-segment.