but I don't want to be in a road train inches in front of a semi when its tire calls it quits.
That doesn't make sense. Being in front of the semi, there's no way it'll hit you when its tire blows. If you're behind it, the computer will notice the semi slowing down and will brake accordingly. Speed matching and collision avoidance is in production cars today, and works well.
The only remaining hazard is tire debris. I think we'll see mandatory tire monitoring systems before we see automated roadtrains, which would mostly eliminate that hazard.
Brake failure needn't be a problem in a road train either: because the cars have very little separation, the speed difference will be small, the failing car will bump into the car in front (after giving a warning through the intervehicle communication system). The car in front can then help bring the failing car to a safe halt.
Total brake failure is very rare, though.