Section 1620. Speed limits on state highways, and on Indian reservations

(a) The department of transportation  with  respect  to  state  highways
  maintained  by the state outside of cities having a population in excess
  of one million, and highways on Indian reservations, may by order,  rule
  or  regulation  establish  higher or lower maximum speed limits at which
  vehicles may proceed on or along such highways than the fifty-five miles
  per hour statutory maximum speed limit.  No  such  maximum  speed  limit
  shall  be  established  at  less than twenty-five miles per hour, except
  that school speed limits may be established at  not  less  than  fifteen
  miles  per hour, for a distance not to exceed one thousand three hundred
  twenty feet, on a highway passing a school building, entrance or exit of
  a school abutting on the highway. Absence of signs installed pursuant to
  this section shall  be  presumptive  evidence  that  the  department  of
  transportation has not established a higher maximum speed limit than the
  fifty-five miles per hour statutory limit.
    (b)  The  department  of transportation, whenever it determines on the
  basis of an engineering and traffic investigation that  slow  speeds  on
  any  part  of  a controlled-access state highway maintained by the state
  outside  of  cities  having  a  population  in  excess  of  one  million
  consistently  impede  the  normal  and  reasonable  flow of traffic, may
  establish minimum speed limits below which vehicles may not  proceed  on
  or along such highway,
    (c)  The  department of transportation may determine the maximum speed
  which may  be  maintained  without  structural  damage  to  bridges  and
  elevated  structures  that are a part of any state highway maintained by
  the state, and, if such maximum speed is lower than  the  maximum  speed
  limit  otherwise  applicable, may by order, rule or regulation establish
  such lower maximum speed limit at which vehicles may proceed on any such
  bridge or structure.