Final answer:
C) The act's primary purpose is to promote and enhance road safety by setting safety standards and providing grants to states for highway safety programs.
The National Highway Safety Act aims to foster road safety by setting nationwide safety standards and offering grants to states for complying with these standards, thus affecting state and local traffic laws and regulations.
Step-by-step explanation:
The National Highway Safety Act's primary purpose is to promote and enhance road safety by establishing safety standards and providing grants to states for highway safety programs. This influences states and local governments by offering federal assistance conditioned on compliance with safety standards, impacting how they regulate items related to highway safety, such as laws against cellphone use while driving.
Legislation dating back to the early 20th century, including the Federal Highway Act of 1921, laid the groundwork for eventual interstate system enhancements designed to guarantee safer and more efficient roadways for both military and civilian use. States and local governments can benefit from federal funding, but in return, they often have to adhere to federal guidelines on issues such as seat belt usage, speed limits, and drinking and driving laws.