In the current NHRA national event design, first adopted in April 2011, each side of the column contains, from top to bottom: a set of blue LED lights (split into upper and lower halves), three large amber lights, one green light, and one red light. At regional and club events, and at national events before 2011, the blue staging lights were replaced by four smaller amber bulbs at the top. Each lane has its own independent column; the two columns face their respective drivers, mounted on a shared fixture spanning the track.
Before a run the driver rolls forward until the car's front tires break a light beam located 7 inches behind the starting line. Interrupting this beam activates the upper half of the blue light, labelled Pre-Stage. The driver then advances a further 7 inches to break a second beam at the starting line itself, activating the lower half of the blue light and the Stage indicator. The front tires must remain behind but on the starting line beam.
Once one driver stages, the other has seven seconds to follow or is automatically disqualified. After both are staged, the automatic system waits up to 1.3 seconds and then triggers the countdown amber lights.
The amber lights can operate in one of two modes. In the Sportsman or Full tree, used for non-professional classes, each of the three large amber lights illuminates consecutively at half-second intervals, with the green light following 0.5 seconds after the last amber. In the Professional tree, used for Top Fuel, Funny Car, Pro Stock, and other elite classes, all three amber lights illuminate simultaneously, followed 0.4 seconds later by the green.
A hybrid variant, called the .500 Professional tree, uses the simultaneous amber flash but extends the green delay to 0.5 seconds. Some bracket classes use this configuration.
Leaving the staged beam before the green light activates causes the red foul light to illuminate in that driver's lane. In professional heads-up starts, only the driver who left first receives the red light if both leave early. The red light does not automatically disqualify a driver; the penalty takes effect unless the opponent commits a lane-boundary violation during the run, in which case boundary fouls take precedence. If a driver leaves before any amber light has illuminated, a more serious infraction is assessed and the opponent is credited with a competition pass regardless.
Credit for the invention is contested. One claim traces the Christmas tree to Wilfred H. David Jr. of the Pel State Timing Association in Lafayette, Louisiana, who reportedly built the first unit in the late 1950s using small glass Christmas tree bulbs and later sold the rights to the Chrondek Corporation. A rival account, published in the NHRA's National Dragster in 2013, holds that the Chrondek Corporation's Oliver Riley developed the step-light countdown in 1962 after discussions with NHRA National Field Director Ed Eaton, with additional input from Dragtronics' Lew Bond. The system was publicly debuted at the 1963 NHRA U.S. Nationals in Indianapolis.
The original 1963 tree had five amber lights and no staging bulbs. Staging bulbs were added for the 1964 Winternationals. For the 1971 season the NHRA introduced the Professional tree format, initially lighting only the fifth amber before the green. In 1986, the tree was redesigned down to three amber lights, a change that also shaved a second off each start sequence in sportsman classes.
NHRA switched from incandescent bulbs to LED lighting at Pomona in 2003, eliminating frequent bulb replacements caused by vibration from Top Fuel exhausts and producing a measurable drop in recorded reaction times due to the faster light output of LEDs. In 2009 a random delay was introduced between the last car staging and the amber lights activating, after competitors learned to anticipate the moment a human starter would trigger the system.
CompuLink has supplied the official NHRA timing system, of which the Christmas tree is one component, since 1984.
Reaction time in drag racing is measured from the instant the green light illuminates to the moment the car's tires clear the starting-line beam. The Christmas tree enables this precise, reproducible measurement. A driver who reacts before the green illuminates commits a foul start; a driver who reacts quickly after the green earns a competitive advantage independent of the car's raw elapsed time.