Sand on the steel wouldn't be a viable solution because you would have to replace the sand every day, and it would be carried away by the water when it would rain. I would agree that the design of the bridge should have been rethought to use something other than steel plates as a road deck. Steel grating could be welded to the plates, and fill the holes with an abrasive compound, but you would have to be concerned about the additional static weight of the deck considering you would be increasing the load.