The Doppler effect (or Doppler shift), named after the Austrian physicist Christian Doppler, who proposed it in 1842 in Prague, is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

The Mayan languages form a language family spoken in Mesoamerica and northern Central America. Mayan languages are spoken by at least 6 million indigenous Maya, primarily in Guatemala, Mexico, Belize and Honduras. In 1996, Guatemala formally recognized 21 Mayan languages by name, and Mexico recognizes eight more. The Mayan language family is one of the best documented and most studied in the Americas. Modern Mayan languages descend from Proto-Mayan, a language thought to have been spoken at least 5,000 years ago.

Reserve Deputy Probation Officers are deputized volunteers who have received special training in order to assist Probation staff in a wide range of supervised responsibilities, that include direct involvement with juvenile an d adult probationers. Reserve Deputies are highly motivated, skilled, and caring people who want to serve their community as a part of the Los Angeles County Probation Department. Reserve Deputies are not a replacement for or an alternative to paid staff, but are intended to be an enhancement to existing resources.

In 2011, the Ohio Turnpike raised its limit to 70 and recorded its lowest traffic fatality rate ever that year. The higher speed limit attracts travelers to use the safer, limited-access, divided highways of the interstates from more traditional roads. That makes both types of roads safer.

A rule change was enacted before the 1974 National Football League (NFL) season to add one sudden death overtime period (15 minutes) to all preseason and regular season games. If no team scored in this period, the game would result in a tie. This rule was enacted to decrease the number of tie games. The first ever regular season overtime, a September 22 game between the Pittsburgh Steelers and the Denver Broncos, ended in a 35-35 draw. It was not until November 10, when the New York Jets defeated the New York Giants, 26-20, that an overtime game would produce a winner.