75 Pa. C.S. § 3363
On highways under their respective jurisdictions, local authorities, subject to section 6109(e) (relating to specific powers of department and local authorities), or the department, upon the basis of an engineering and traffic investigation, may determine that the maximum speed permitted under this subchapter is greater or less than is reasonable and safe under the conditions found to exist upon any such highway or part thereof and establish a reasonable and safe maximum limit. The maximum speed limit may be made effective at all times or at times indicated and may vary for different weather conditions and other factors bearing on safe speeds. No maximum speed greater than 55 miles per hour shall be established under this section except on highways listed in section 3362(a)(1.1) (relating to maximum speed limits), where the maximum speed for all vehicles shall not be greater than 70 miles per hour.
75 Pa.C.S. § 3363