It's called a Botts' Dot... "a round nonreflective raised road marker that provides tactile feedback to drivers when they move across designated travel lanes"... as described by Wikipedia. In 1953, Dr. Elbert D. Botts, working in the Caltrans materials testing lab in Sacramento came upon the idea of using a raised pavement marker to help make the painted lines separating lanes last longer. They also alert motorists when they drift out of their lane. There are an estimated 30 million Botts' Dots in place today on California freeways and highways -- a lasting legacy to Mr. Botts, who passed away in 1962.

~ the whole story ~