Botts' dots

Botts' dots are round non-reflective raised pavement markers. Botts' dots may also be called turtles (Washington and Oregon) or buttons (Texas and other southern states). In many parts of the US and 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 moving across designated travel lanes, and are analogous to rumble strips.

Botts' dots are named after Dr. Elbert Dysart Botts, a California Department of Transportation (Caltrans) engineer credited with overseeing the research that led to the development of the markers.

Botts' dots are most commonly white but may be yellow when used to substitute for the yellow lines that divide opposing directions of traffic in North America. The dots are made of various ceramic materials or plastics like polyester.

On some roads, lanes are marked only with a mix of Botts' dots and conventional reflective markers, eliminating the need to repaint lane divider lines. Botts' dots are rarely used in regions with substantial snowfall, because snow plows damage or dislodge them.

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 of glass and were attached to the road by nails or tacks, as suggested by Botts. The nails were soon abandoned: his team discovered that when the dots popped loose under stress, the nails punctured 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 protégés was responsible for the epoxy.

In September 1966, the California State Legislature mandated that Botts' dots be used for lane markings for all state highways in all non-snowfall areas. Today, there are more than 25 million Botts' dots in use in California. In California, highway lanes may be marked either solely by Botts' dots, or dots placed over painted lines. Four dots are used for broken lines on freeways, and broken lines on surface streets may use only three dots. Reflective Stimsonite pavement markers are placed at regular intervals between Botts' dots to increase the visibility of lane markings at night. In the Las Vegas area of southern Nevada, roads with multiple lanes use four pavement markers for each broken white line, a reflective Stimsonite marker followed by three Botts' dots.

...
Wikipedia