The history of Caltrans and its predecessor agencies during the 20th century was marked by many firsts. It was one of the first agencies in the United States to paint centerlines on highways statewide; the first to build a freeway west of the Mississippi River; the first to build a four-level stack interchange; the first to develop and deploy non-reflective raised pavement markers, better known as Botts' dots; and one of the first to implement dedicated freeway-to-freeway connector ramps for high-occupancy vehicle lanes.