The good news is that it was the rear tire. I was driving on highway 10 in heavy rain with moderate traffic, so if a front tire had blown there would have been greater danger of losing control. As it is all I felt was a lot of vibration, like I was driving on rough asphalt. I managed to get to the side of the road, and call CAA (AAA for my USA friends).
Because I had CAA-Plus I could have gotten towed all the way to the Eastern Townships and have the tire replaced there. Unfortunately none of the garages contacted had the tire in stock. So I had to double back to Montreal to see my tire-specialist garage here. I am sitting in their waiting area as I write this.
So much for the start to my holiday -- I was going to spend the afternoon with brother bishops at our gathering in Sherbrooke, and the evening with Archbishop Luc Cyr, but alas. Still, it could have been worse -- my plan was to drive tomorrow from Sherbrooke through the White Mountains to Maine, and I'm certainly glad the tire didn't blow while I was in the middle of nowhere. And nothing says I can't take a break and have fun in my hometown -- once I get out of here, of course!