Specialist time-trial bikes can be dangerous. This is especially true if they are used by amateurs in emulation of the pro tour.

Many amateurs ride solo, but there is no professional model of a 'solo-ride bike' other than time trial bikes, which would be deadly on roads with cars. 

If the UCI governed grand tours included time trial stages in which the pros ride road bikes (ideally without aero bars, which again result in danger) but with other modifications allowed, these stages would be less dangerous for the pros and it would show amateur riders the optimal set up for the sort of riding that many (solo) amateurs do.

Chris Froome recently proposed that Time Trials be held on road bikes for reasons of safety.

Photo: Christopher Froome during individual time trial stage 9 of Tour de France 2012 by Igs165