I think the longest stretch of road without civilization or water you are likely to find in the western US will be somewhere around 50 to 70 miles. I took a route from San Diego, across the desert, up through the 4 corners, and across Kansas. I carried about 7 litres of water in the desert, because well it's desert and hot. But then I ditched some of my extra water going over the Rockies, and ended up running out in Eastern Colorado and again in Kansas, which is almost as hot, and just as desolate as the low desert. After the desire to breathe, I think the desire for water is about the strongest human urge there is. Anyway, I may have drunk from some discarded pop bottles on the side of the road. Watch out for the yellow Pepsi though.