Montana: Montana Highway Patrol, Helena, MT. 406-444-3300.
A person operating a loaded vehicle upon a public highway for the purpose of transporting solid waste as defined in 75-10-203, except a commercial motor vehicle or a vehicle transporting unprocessed agricultural products, shall attach, cover or otherwise secure the load sufficiently to prevent littering or creating an obstruction dangerous to the public traveling on the highway.
75-10-203 § (11)(a)
“Solid Waste” means all putrescible and nonputrescible wastes, including but not limited to garbage; rubbish; refuse; ashes; sludge from sewage treatment plants; water supply treatment plants; air pollution control facilities; construction and demolition wastes; dead animals, including offal; discarded home and industrial appliances; and wood products or wood byproducts and inert materials.
Each city or county may impose and enforce regulations concerning the tarping of a load.