You only need the large gap once you've accelerated up to a much higher speed. Going from small gap while standing in lights to large gap at a higher speed can

You only need the large gap once you've accelerated up to a much higher speed. Going from small gap while standing in lights to large gap at a higher speed cannot be reached in unison. The later cars must inevitably accelerate slower or the first cars must accelerate faster in order for the larger gaps to appear. So even if all cars did in fact start rolling at the exact same time when the light turns green the acceleration of any tailing car is bounded by some safety margin as a function of speed.