Well, probably a waste of time carrying the belt anyway. They rairly fail. If you have replaced at say 50,000 miles its good for another 50,000 or more.
The belts fail usually because the idler or tensioner or waterpumps have failed, and destroyed the belt.
You would also have to carry i think a 15 MM wrench to rotate the tensioner to put a new belt on. But if a bearing failed, you woulld just destroy the new belt.
So change your belt, check the feel of the bearings on the idler,tensioner, waterpump. if any are not smooth,replace.