Because Guam doesn’t have any natural sand, but rather coral, the island nation makes its asphalt using a mix of ground coral and oil rather than buying sand from abroad.

Local roads are usually paved by a coral/oil mixture, so when it gets wet, the oil tends to float to the surface of the roads, thus making them dangerous. That's why the speed limits on most of the island is about 35 mph.

More Info: en.wikipedia.org