Paul and all,
My friend with a Roaring 40's GT asked me to help answer some of the questions re: drag strip timing. My name is Dave McClelland and I have been announcing races for some 50 years....NHRA National Events since 1961. Have watched lots of runs!!!
First the track...1320 feet from starting line to finish line. Timing is controlled by infrared beams across the track at the starting line, 60', 330', 660' or 1/8 mph; both elapsed time and speed, 1000' e.t., and quarter mile e.t. and speed.
The clock is not controlled by the starting line system, or christmas tree. That device is used to actually motivate the drivers into leaving the line. The clocks begin operating the moment the front tire leaves the starting line beam.
The question regarding reaction time is answered by noting the tree has nothing to do with the timing of the cars. The yellow light is either 3 bulb or single...depending upon the category of competition, at least here in the states. In the full or 3 bulb mode, each yellow light is on for .5 of a second before the tree goes green. That is used in sportsman competition, except T/AD, TA/FC, Super Comp, Super Pro and Super Street. It is designed to allow the driver to anticipate the actual green light...as opposed to the single bulb, or Pro Tree used by T/F, F/C, Pro Stock and P/SMcycle/ That is generally referred to as a reaction starting device. Currently, stateside the duration is .4 seconds...as opposed to the .5 that was used up until a few years ago.
In the case of the full tree...you are not responding to the green light, but launching on the second or last yellow light...depending on how your car leaves the line. Reacting to the yellow is also possible in the Pro start mode, if both car and driver are quicker in reaction that the length of time
(either .4 or .5) of the yellow light. Pro Stock Motorcycles immediately come to mind...and that also explains the number of red lights in that category.
Bear in mind, the reaction time readout is in reality, a combination of both the driver's R.T. and the way the car responds to the launch. That's why so much emphasis is made on getting the car to work better.
There have been changes in recent times, made to the way reaction time is displayed on the screen. If the screen or printout shows a .510...that indicates the driver had a light of .010 before red lighting...anything .499 and under is a red light. If the screen showed a .010...that would be representative of the later type of display...but revealing the same reaction time. That would mean anything reading .999 or less is a red light in that mode.
As the car makes its way down track, the breaking of the infrared beams triggers actions in the timers, giving the readout for the distances mentioned above. This allows the driver/tuner/crew chief to determine the consistency of the car...the place where work is needed to improve performance...or in some cases, tear their hair out because nothing seems to work!!!! The latter is more frequent that you would imagine.
One major point to remember...each individual lane is timed separately...thus leading to the situation where the quicker car can lose the race to a slower one because of better reaction time. That's why driver and crew work so hard to improve on reaction times. Bear in mind, if one car leaves first, the other could sit there for an hour and the clock would not start until it leaves the line.
Hope I haven't bored you with all of this but it's about as concise as I could make it and still try to explain how it all works. When you are announcing in the tower, you get the sight of the progressive times coming up on the screen, so thus you can sound like you really know what you're talking about!!!
My best to all and stay safe.
Dave McClelland