They both can apply. The flow of traffic laws, such as the one in California, are to prevent people from going significantly below the speed limit. Its a better approach, in my opinion, then a minimum speed limit law that many states use.

That said, you'll end up driving white-knuckled and fearful of your life if you dare go the speed limit on the Mass Pike. You'd have to drive 70-75 minimum here just to feel safe.