Pennsylvania MUTCD

Interstate Route Marker (M1-1)

Justification:  The Interstate Route Marker shall be authorized for use to mark interstate routes and intersecting highways approaching an interchange with an interstate route.
Size:  The 36-inch by 36-inch size for one and two digit route numbers and the 45-inch by 36-inch size for three digit route numbers shall be used on the interstate route.  The 24-inch by 24-inch size for one and two digit route numbers and the 30-inch by 24-inch size for three digit route numbers shall be reserved for use on intersecting highways.
Pennsylvania places the state name in the shield; however, there are some shields without the state name which are usually installed by contractors and not PennDOT.

US Route Marker (M1-4)

Justification:  The US Route Marker shall be authorized for use to mark US traffic routes.
Size:  The standard size of M1-4 shall be 24-inches by 24-inches size for one and two digit route numbers and the 30-inches by 24-inches for three digit route numbers.  The 36-inch by 36-inch size for one and two digit route numbers and the 45-inch by 36-inch size for three digit route numbers may be used on expressways.

Pennsylvania Route Marker (M1-6)

Justification:  The Pennsylvania Route Marker shall be authorized for use to mark Pennsylvania traffic routes.
Size:  The standard size of M1-6 shall be 24-inches by 24-inches for one and two digit route numbers and the 30-inches by 24-inches for three digit route numbers.  The 36-inch by 36-inch size for one and two digit route numbers and the 45-inch by 36-inch size for three digit route numbers may be used on expressways.  Pennsylvania uses the state symbol, the Keystone, for highway marker background.  The Pennsylvania Turnpike Commission uses green on white keystones with "PA TURNPIKE" at the top on the Amos K. Hutchinson Bypass/PA 66 and the Mon-Fayette Expressway/PA 43.  Standard markers with "TOLL" at the top are still used on the James E. Ross Highway/PA 60, and were used on the other two expressways before being replaced because the "toll" connotation made drivers resist using the expressway according to the Turnpike Commission.