Just the other day we had to buy tires for our utility trailer. The manager of the tire store said he could only sell us ST tires since they were for a trailer.
I've been searching the internet for a PA law that says we must use ST tires on trailers, but can't find anything. All I could find was this:
"Tire standards. A trailer shall have tires that were manufactured in conformance with standards contained in Chapter 159 (relating to new pneumatic tires). See 75 Pa.C.S. § 4525 (relating to tire equipment and traction surfaces)."
We did end up with Towmasters (ST). We'll see...