they will certainly cut down on the spray off of your own vehicle, not going to do much for the spray from everyone else. Will also keep the debris from your car down lower.
iirc @UtahJoe has wrapped his bikes in a tarp for this reason. . . .
this is one of the many joys of having a pickup, just toss the bike in the bed (laying down usually as being under the cover is additional protection) and its not a concern.