If we accept your premise that the government can spy on you simply because an activity is regulated, then the Fourth Amendment is effectively dead. Under that logic, the state could mandate interior cameras in every heavily regulated business, or search the backpack of every passenger using public transit without a warrant.
You have a reasonable expectation of privacy for the contents of your trunk, your backpack, and your travel history. The police cannot search your trunk without a warrant just because you are driving on public roads. They should not be able to search your travel history either.