Tires: tire plug and safety, rubber manufacturers association, industry trade group

Question
I had a tire plug put in to seal a nail hole in a radial tire Goodyear Assurance tripl tre.  The plug is right at the edge of the tred.  The tire has plenty of tred left.  
How safe is this?  Am I risking a blowout at highway speeds?

Answer
Keith,

If you have described this accurately, the tire is not repairable.

The Rubber Manufacturers Association is an industry trade group of companies that manufacture products out of rubber - and that includes tires.  They have committees with the best minds available to discuss and publish advisories on various sujects - including repairing tires:

https://www.rma.org/publications/tire_service_professionals/index.cfm?PublicationID=11303

If you download this document, you will see the repairable area - and it sounds like your tire is not repairable.  Repairs in this area tend to cause the tire to delaminate - usually in the form of a belt leaving belt separation - commonly called a tread separation.  This things can sometimes have tragic results.

Secondly, you'll notice that a proper repair includes a plug AND patch.  Plugs tend to leak by themselves and provide no structural reinforcement over the hole created by the puncturing object.

All in all, this sounds like you should replace the tire for safety's sake.