On his website, trucking industry expert witness Lew Grill provides links to the Federal Motor Carrier Safety Regulations, including Compatibility of State Laws and Regulations Affecting Interstate Motor Carrier Operations:
General applicability and definitions
ยง355.1 Purpose.(a) To promote adoption and enforcement of State laws and regulations pertaining to commercial motor vehicle safety that are compatible with appropriate parts of the Federal Motor Carrier Safety Regulations.
(b) To provide guidelines for a continuous regulatory review of State laws and regulations.
(c) To establish deadlines for States to achieve compatibility with appropriate parts of the Federal Motor Carrier Safety Regulations with respect to interstate commerce.
http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrguidedetails.aspx?rule_toc=732§ion_toc=732