Len, I have to disagree, I have the hydramaster ones, and use them when I have to cross a footpath. They are about 3 or 4 feet in width. The length doesnt matter as long as they are not too steep. I think that if you put them down with or without a sign, you would be covered in case of an accident. They are bright yellow and black stripped, so are very visible.
As an aside I made the holes slightly wider for the hose's to make them easier to place flat.