The traffic is backed up, but, aside from saying this, can we say the road is backed up instead? If not, why? Thanks for helping.
Yes and no.
Usually we would say "the road is backed up with traffic".
It's easy to find examples by Googling "road is backed up"
The Anthony Henday at Lessard Road is backed up with cars and tow trucks in Edmonton
"To back up" and the noun "backup" referring to traffic jam seems to be American English.