All the routes that the POTUS will be traveling should be examined and tested for trafficability for all the vehicles in the motorcade.
Any road that can't handle the motorcade is excluded from the list of primary, secondary, ... routes that the USSS plans out.
Nothing is assumed, presumed, or untested.
They may not be fired, but they won't be working protective services anyway. And rightfully so. This is an inexcusable error in judgment and implementation.
Could you imagine if there was an incident and the POTUS' beast could not leave the area??