Before each race (known as a pass), commonly each driver is allowed to perform a burnout, which heats the driving tires and lays rubber down at the beginning of the track, improving traction. The cars run through a “water box” (formerly a “bleach box”, before bleach was replaced by flammable traction compound, which produced spectacular, and dangerous, flame burnouts; the hazard led NHRA to mandate use of water in the 1970s [2 ]).
Modern races are started electronically by a system known as a Christmas tree, which consists of a column of lights for each driver / lane, and two light beam sensors per lane on the track at the starting line. Current NHRA trees, for example, feature one blue light (split into halves), then three amber, one green, and one red.  When the first light beam is broken by a vehicle’s front tire (s), the vehicle is “pre-staged” (approximately 7 inches (180 mm) from the starting line), and the pre-stage indicator on the tree is lit. When the second light beam is broken, the vehicle is “staged”, and the stage indicator on the tree is lit.  Vehicles may then leave the pre-stage beam, but must remain in the stage beam until the race