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As it applies to drag racing, the tree is a strand of different colored lights positioned between two lanes, 25' ahead of the starting line. Each lane has its own side of the tree, with it's own set of lights.
The top area of the tree is where the racers prestage and stage their cars. The next set of three amber lights count down after both racers have staged. The next bulb down is green, a lights if the racer did not leave too early. The bottom light is red, and lights if the racer committed the foul of leaving early.
The point of the tree is for reaction time, which is a major factor of a drag race. The goal is for the car to leave quickly after the green light comes on. If the car crosses the beams before the green light comes on, he red light fouls.
Because it takes a fraction of a second for the eye to see the light, the brain to process it and command the body, and for the car to react and cros the starting beams, most racers anticipate the green by leaving when they see the bottom amber turn on.
In certain classes of racing, cars of different ETs race each other. In these races the slower cars will leave sooner by the differences in projected (or declared) elapsed time, so that the faster car catches him by the finish line. To accomplish the staggered start, the different sides of the tree will start the count down at different times.
There are three different timing sequences of the tree, based on the class of racing. a Pro Tree has all three ambers lights flash together 4/10 of a second before the green. A 4/10 Sportsman tree has a 4/10 second delay between the first amber light an the next one below it to the green. A .500 Sportsan tree has a 1/2 second between Ambers to the green.