This illustration shows one of the problems with boulders, based on some actual tests. An 85-ton truck, traveling at 20 miles per hour, and impacting the boulders at a 20 degree angle, would strike three or four boulders. Some of these boulders would roll up under the front carriage and it would push them along the ground. The large brown line here could represent the drop-off, the edge of the road. The truck traveled a total distance of 80 feet under these conditions. Which means that these boulders, to actually stop that vehicle from going over the edge, would have to be placed back from the edge about 22 feet. If we place boulders right on the edge, all we’re going to do is take some of them off the edge with us.