I have not thought much about these things over my 11 years of riding. But, I do know they are a hazard. One day I was riding up to Sunrise on Mt Rainier and that road has some great twisties. Well, it also has a lot of tar snakes. It really sucked. I could feel my front slip several times. But as I said over the years I can't recall worrying too much (or even seeing these things) and I have ridden from Seattle to Montana many times, I've been to Reno from Seattle a few times and I go into Canada as well. Maybe certain locales or state DOTs use tar snakes more than others? But I do agree they suck and we need to be cautious out there on them. The road to Sunrise is in the Mt Rainier National Park and maybe they use these things to keep the road in ok shape since its tough to repave and costly in a National Park.
Scott