N.H. Rev. Stat. § 236:7
Whenever the commissioner of transportation shall determine upon the basis of an engineering and traffic investigation, or in the event of vehicle or weather emergencies, that any prima facie speed limit set forth in RSA 265:60 is greater or less than is reasonable or safe under the conditions found to exist at any intersection or other place, or upon any part of a class I, class II or class III highway, outside the compact part of cities or towns, said commissioner may determine and declare a reasonable and safe prima facie speed limit thereat which shall be effective when appropriate signs giving notice thereof are erected at such intersection or other place or part of said highway. Such signs may carry either a fixed speed limit legend or a changeable message so designed as to permit display of different speed limits at various times of the day or night. Such a prima facie speed limit may be declared to be effective at all times or at such times as are indicated upon said signs. The commissioner shall keep and maintain a full and complete record of all speed zones established by him and all alterations, amendments or removal thereof.
RSA 236:7
1949, 286:3. 1950, 5:1, part 9:1, par. 2. RSA 249:11. 1981, 87:1. 1985, 402:6, I(b)(7). 1999, 73:3, eff. July 27, 1999.