You may be making reference to this?
http://www.dot.ca.gov/hq/planning/ae.../puc051107.pdf
The PUC is just another code or tool such as the penal code or vehicle code. There are 29 codes in California. A police officers authority to enforce the law is governed by the penal code.
As I said before, the police may also enforce federal law. It is just a mater of filing the cases with the proper agency, DA or US Attorney.