Section 1181. Minimum speed regulations

(a) No person shall drive a motor
  vehicle at such a slow speed as to  impede  the  normal  and  reasonable
  movement  of  traffic  except  when  reduced speed is necessary for safe
  operation or in compliance with law.
    (b) Whenever a minimum speed limit has been established as  authorized
  in  sections  sixteen  hundred  twenty  or sixteen hundred forty-two, no
  person shall drive at a speed less than such minimum speed limit  except
  when entering upon or preparing to exit from the highway upon which such
  a  minimum  speed limit has been established, when preparing to stop, or
  when necessary for safe operation or in compliance with law.