Mont. Code Ann. § 60-7-204

A person who owns, controls, or possesses livestock may not herd or drive more than 10 livestock on an interstate or state primary highway unless the livestock is preceded and followed by flag person escorts for the purpose of warning other highway users. Livestock may not be herded or driven on an interstate or state primary highway during nighttime, as that term is defined in 1-1-301, except in a case of emergency. In the case of an emergency during the nighttime, the flag person escorts shall use adequate warning lights, such as but not limited to portable lamps, lanterns, or rotating beacons. This section does not apply during daytime at posted livestock crossings on highways.