Thanks for sharing this (and for re-posting to the forum instead of email). I can't say that I've seen "petroglyphs" like these before. Taxpayer dollars well spent supporting the arts?
Based on the location, one guess is that a road repair crew may have been using up the extra tar at the end of the day -- on the way down the hill after working on the larger nearby highway. (Or, testing its consistency on the way up?) There are a few roads that go up to the main highway, but it seems this one isn't used as much, which might have made it a good canvas. From the different images, it does not appear that they were all done at the same time. Hard to photograph from angle of creation. I tried to photograph them in such a way that I could capture the entire image. At first I thought they were just hanging off the back of a truck dripping the tar. Some of the designs (like the whale) are too complex for that--they had to have stopped to create them.
Another idea is that if someone had a roof repaired in the area, the creator/s could have spent the afternoon "painting." The art is distributed over a distance, though, so my first hypothesis about a moving truck --or a least a way to make the tar portable is more likely.
This is a fantastic discovery. Thanks for sharing!