Botts' dots are round nonreflective raised pavement markers. In many U.S. states and in several other countries, Botts' dots are used (along with reflective raised pavement markers) to mark lanes on highways and arterial roads. They provide tactile feedback to drivers when they move across designated travel lanes, and are analogous to rumble strips.
Botts' dots are most commonly white but may also be yellow when used to substitute for the double yellow line that divides opposing directions of traffic in North America. The dots are made of various ceramic materials, or plastics like polyester.
On some roads, all lanes are marked only with a mix of Botts' dots and conventional reflective markers, eliminating the need to repaint lane divider lines. Along with their reflective cousins, Botts' dots are rarely used on freeways in regions where it snows, because snow plows scrape them off.
Caltrans engineers may have studied the concept of raised pavement markers as early as 1936. However, the department did not commence research in earnest until 1953, when the postwar economic boom resulted in an alarming increase in the number of cars and car accidents in California. Painted lines tended to become invisible during rain.
The initial dots were made out of glass and were attached by nails or tacks to the road, as suggested by Botts. The nails were soon abandoned; his team discovered that when the dots eventually popped out under stress, the nails would puncture tires. Contrary to a common myth, the published record does not make clear whether Botts invented the famous epoxy that solved the problem; some sources indicate that one of his proteges was responsible for the epoxy.
In September 1966, the California State Legislature mandated that Botts' dots were to be used for lane markings for all state highways except in areas where it snowed in the winter. Today, there are more than 25 million Botts' dots in use in California. In California, highway lanes may either be marked solely by Botts' dots, or dots placed over painted lines.
More recently, Botts' dots have been used in the snow-free areas of states other than California, most notably: Arizona, Washington, Hawaii, Louisiana, Mississippi, Nevada, and Texas. Some states that do experience snow, particularly Pennsylvania and Massachusetts use Botts' dots during the summer months for temporary lane markings in construction zones. Typically, the dots are installed when construction starts in the spring, and they are removed when work stops for the winter months.