I just got back from a rather long road trip. We drove from my island in the Pacific half-way across North America to attend a wedding. The trip there we did in two very very long days (we took a more civilized three days to drive back with a day off in the middle). Once we hit the prairies on the second day there was nothing but flat farm lands as far as I could see and I was bored of being in a car. So I set my mind to pondering my surroundings which mostly consisted of sky, prairie and the road we were on.
We were barreling down a major highway that was also a thoroughfare for convoys of tractor-trailers. Since these trucks haul heavy loads, when they brake all that momentum from their load goes into the highway surface, slightly pushing down the highway beneath the tires and creating a bump in front like a really hard version of a cushion. If one was to push down on the cushion with your hand, the cushion would depress beneath the hand and raise up around it. On the highway, this effect builds up over time creating dips and bumps, and since the trucks don’t break all that the same place, a wash board effect can develop along the highway surface. Cars contribute to the same effect, but since they weigh a fractions of a fully loaded tractor-trailer their effect is minimal. I’ve done a lot of long distance highway driving and experienced this effect of driving on a what feels like a wash board many times.
What we noticed this time was dark splotches on the highway just after each bump – the larger the bump the darker and larger the splotch. I’m guessing the dark splotches are oil, the cumulation of little drips shaken loose by the bump from thousands of passing cars. In the July 2010 Scientific American, I found the statistic that the average annual spillage of oil from natural seams and human activities such as transportation is 380 million gallons globally – I wonder how much of that comes from oil drips released by bumps in the road?