Mont. Code Ann. § 61-3-711

It is the policy of this state to promote and encourage the fullest possible use of its highway system by authorizing the making and execution of reciprocal or proportional registration agreements, arrangements, and declarations with other states, provinces, territories, and countries with respect to motor vehicles, trailers, semitrailers, or pole trailers registered in this and other states, provinces, territories, and countries thus contributing to the economic and social development and growth of this state.