It's not the roads, it's the loads. Unlike hazardous liquids, or even food grade like chocolate or sweeteners, water is not as "touchy" with the loading and unloading process. Therefore, it's much easier to get wet, especially when unloading. Sometimes the trailer doesn't completely empty out, and we sometimes get a leg bath when we pull the hose.
Now we could close the valve when we detach, but if we were to forget to open it back up and it freezes shut, were screwed and can't reload until we thaw it back out.