You know how the road ahead of you appears on the highway? There’s a dark broad stripe in the center of the road, a pair of lighter-colored stripes adjoining it on either side, and then another pair of slightly darker stripes adjoining them. Roughly, GRAY-WHITE-BLACK-WHITE-GRAY.
The highway, when it’s new, is a uniform color, I presume. So how does such a pattern present itself? Is the dark center stripe the result of exhaust? (But exhaust pipes are placed in various locations, and very rarely in the exact center of the car). Are the two lighter stripes (that I’m calling WHITE) caused by the tires wearing away debris? (But wouldn’t the tires themselves tend to leave dark marks, rather than lighter marks?)
Anyone got a clue, or a source?
Concrete or asphalt*?
On concrete, the very dark center section is from dripping oil. (Note that when there is a bump in the road the dark patch will become even more noticeable as more oil and greasy dirt is shaken loose when the car hits the bump–which makes it a good way to look for unannounced bumps in the road).
The less dark sections are where simple dust/dirt is accumulating over time.
The lightest sections are where tires are constantly blowing the dust out of the way so that it does not accumulate.
On asphalt, the same basic rules apply, with the added factor of the asphalt composition. Depending on the source and variety of paving, (prepared asphalt, chip-and-seal, etc.), the tire wear areas will eventually become shiny from being constantly rolled over and, in some cases, the squishy oil in the asphalt will be squeezed out from between the stones in the mixture, leaving asurface that has greater stone content–with limestone and other light colored varieties predominating.
- (In British-speaking regions, substitute tarmacadam or its derivatives for asphalt.)