Annefan: You are correct, and I was wrong.
I did a little research and found the answer in §3368(c)(2) of the following PA statute:
"§ 3368. Speed timing devices.
(a) Speedometers authorized.--The rate of speed of any vehicle may be timed on any highway by a police officer using a motor vehicle equipped with a speedometer. In ascertaining the speed of a vehicle by the use of a speedometer, the speed shall be timed for a distance of not less than three-tenths of a mile.
(b) Testing of speedometers.--The department may appoint stations for testing speedometers and may prescribe regulations as to the manner in which the test shall be made. Speedometers shall have been tested for accuracy within a period of 60 days prior to the alleged violation. A certificate from the station showing that the test was made, the date of the test and the degree of accuracy of the speedometer shall be competent and prima facie evidence of those facts in every proceeding in which a violation of this title is charged.
(c) Mechanical, electrical and electronic devices authorized.--
1. Except as otherwise provided in this section, the rate of speed of any vehicle may be timed on any highway by a police officer using a mechanical or electrical speed timing device.
2. Except as otherwise provided in paragraph (3), electronic devices such as radio-microwave devices (commonly referred to as electronic speed meters or radar) may be used only by members of the Pennsylvania State Police.
3. Electronic devices which calculate speed by measuring elapsed time between measured road surface points by using two sensors and devices which measure and calculate the average speed of a vehicle between any two points may be used by any police officer.
4. No person may be convicted upon evidence obtained through the use of devices authorized by paragraphs (2) and (3) unless the speed recorded is six or more miles per hour in excess of the legal speed limit. Furthermore, no person may be convicted upon evidence obtained through the use of devices authorized by paragraph (3) in an area where the legal speed limit is less than 55 miles per hour if the speed recorded is less than ten miles per hour in excess of the legal speed limit. This paragraph shall not apply to evidence obtained through the use of devices authorized by paragraph (3) within a school zone."
