Mont. Code Ann. § 60-2-141

Unless prohibited by federal law, all contracts let by the commission for the construction or reconstruction of the highways located on commission-designated highway systems and state highways, including portions in cities and towns and all contracts entered into under 7-14-4108, must use Montana-made wooden guardrail posts, fenceposts, signposts, and decking when appropriate and when the cost of wooden materials is less than or equal to the cost of other materials.