N.H. Rev. Stat. Ann. § 235:37

The commissioner of transportation shall require every private contractor engaged in construction or maintenance work by contract with the state, county, city or town, on any state, state aid or town aid highway or bridge project, and who employs 5 or more persons in the state, to file with the commissioner of labor and commissioner of transportation of this state a declaration of acceptance of the provisions of RSA 281-A, "Workers' Compensation" before any work is begun on such project.