Legal Question in Traffic Law in California

In theory, there is a constitutional law concept called "federal preemption" in which a state law that is contrary to federal law is invalid because when there is such a conflict of law, federal law trumps state law.

However, the 10th Amendment gives a broad grant of power to local governments such that, especially in the area of regulatory and municipal law (as in the case of a law regulating vehicles such as cars or bicycles), a state law is given deference to regulate these local matters. Also, the Feds do not typically go to every state and arrest the California police officers that are enforcing state law, especially municipal and county ordinances such as the vehicle code. It is the reason that we have medicinal marijuana in CA. The federal law is that marijuana is illegal for all purposes, but in practice, the California police simply enforce CA law so that medicinal marijuana use is not prosecuted as a crime in this state.