An excavation performed exclusively for the lawful construction, reconstruction, or maintenance of a Class I, II, III, IV, or V highway. A copy of the pit agreement executed between the pit owner, the agent, and the governmental unit shall be filed with the Planning Board prior to the start of excavation. In addition, the provisions of § 170-40 of this chapter and the provisions of RSA 155-E:2, IV(b) and (c), shall be complied with.