N.H. Rev. Stat. Ann. § 237:15

Any excess of toll revenue which may exist from time to time in any of the individual turnpike reserve accounts or the New Hampshire turnpike system reserve account shall be used only to finance extension studies, maintenance, construction, reconstruction and extensions of the system wherever located, interest on bonded indebtedness or retirement of bonded indebtedness or other costs which may be properly charged against these accounts or to the New Hampshire turnpike system.