In my plan we have vehicles bridges made of steel. When they get wet they become slippery and dangerous for cars.
Is there a standart rough metal plate that can be welded to the existing plates, so the bridge is being treated with minimum closure time?
You might want to post this question in the structural engineering, or civil engineering forum. I know there are coatings that can be applied to steel in order to increase the slip coefficient in wet conditions. I don't know how they would hold up under the loads put on them from high vehicle traffic.
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.
Even steel grate is slippery when wet. Any application on steel decking would not last long from traffic,therefore, the best solution is speed regulation. Grated steel decks are in my opinion a good choice, since the material can last decades with nominal maintenance .