i personally would never canter on the road, unless i couldn't help it, but i would try & stop them. i see where you're coming from about trotting tho. i don't trot for long periods of time when on the road anyway, so no experience of concussion.
i think no matter what pace you do on roads, your horse will get concussion eventually. just the faster the gait, the shorter amount of time it takes for the horse to get concussion, & likewise, the slower the gait, the longer it takes for the horse to get concussion.