Answer:
the government's sovereign immunity
Explanation:
In the US, the federal and state governments have sovereign immunity which means that they cannot be sued unless they agree to it. In the US, the federal government waived their immunity protection from a series of possible torts through the Federal Tort Claims Act. But that law does not include litter or accidents occurring in highways.
Sovereign immunity basically states that the federal government cannot be sued for its actions unless those actions are included in the Federal Tort Claims Act. To be able to sue a state government other rules apply, specially regarding the circumstances around the reason for the claim.
