The New Hampshire turnpike system shall be limited access highways as defined in N.H. Rev. Stat. § 230:44, except that N.H. Rev. Stat. § 230:52 shall not apply to ducts, pipes, pipelines, poles, wires or other structures permitted under N.H. Rev. Stat. § 237:5, II(g) or to pre-existing facilities on highways, not previously restricted as to access, used as toll-free sections of the system.