The idea of a federal agent stopping you for a traffic infraction is insane on its face. That'd be very rare, if not unheard of, in normal times no? How would they charge you? Are there federal laws on the books for speeding or not wearing a seatbelt?
Even worse feds will use local cops as fodder to pull over actual murderous criminals on traffic infractions, not knowing what they are dealing with. They then let the local cops take the risk and come by with their meal team 6 squad afterwards.
Look into "dual sworn" officers. Although I've seen a few investigations which show that the federal officers will just send a text message, on a private phone, to uniformed officers when they want them to "check something out."
The US Park Police can and do enforce basic traffic regulations in national parks - which includes some roads, like the Blue Ridge Parkway, that are “linear parks”. In respect of the fact that these roads are often used by local traffic, they will generally permit things that would be legal under the law of the state you are in (e.g., concealed firearm carry, so long as it’s in the car and not brought into a ranger station or other building on the federal property).
