Written by Gregory Monte.
This is an important question to answer, because I have seen online inquiries about whether the police can enforce stop signs in parking lots. This blog post will focus on Pennsylvania, but most states have similar laws on the books.
There are two sections of the Motor Vehicle Code which provide insight.
First of all, businesses and private citizens are not allowed to place stop signs on their own.Title 75 Section 6125(a) says: “No person shall place, maintain or display upon or in view of any highway any unauthorized sign…”
Secondly, local governments/municipalities must place stop signs by official ordinance. Title 75 Section 6109(b) says it “shall be by ordinance of the local governing body…”
So, if the stop sign was authorized by a local ordinance, then it can be enforced. An open public records request to the local municipality would uncover this information.
A stop sign that was not properly authorized and placed is illegal and is not enforceable. My son won his stop sign case precisely on this argument. He quoted the very definition of a traffic-control device as stated in Title 75, Section 102 and the judge agreed:
“Official traffic-control devices.” Signs, signals, markings and devices not inconsistent with this title placed or erected by authority of a public body or official having jurisdiction, for the purpose of regulating, warning or guiding traffic.
You can’t enforce a traffic-control device unless it is “official.”
Traffic Ticket Defense Blog 