The 80% statistic is pretty impressive, and is actually true, no matter how illogical or imaginary biorhythms may be! However, it does not follow out of any mystical cycle properties, but out of pure probabilities. Let's do the numbers:

A critical day is the day when the biorhythm wave crosses the horizontal axis, and also the days after or before. Thus, six days are critical out of every (any!) cycle you may invent.
The odds of someone NOT being on a critical day in the physical (23 days) cycle are 17/23, and the similar odds for the emotional and intelligence cycles are 22/28 and 27/33. The odds of someone NOT being on ANY critical day are then 17/23 times 22/28 times 27/33, which comes to about 47.5%.
If two drivers are involved in a crash, the odds of BOTH not being on any critical day are 47.5% times 47.5%, or 22.6%. So, finally the odds of SOMEONE being in any critical day are 77.4% -- a quite good match for 80%!
Of course, if more drivers were involved, this number goes up and eventually becomes almost 100% sure that SOMEONE will be on a critical day, but let's keep to two drivers, which matches the given odds.
In conclusion... the statistics about 80% are pretty logical and probable, just because of pure mathemathics, without any need of biorhythmic cycles! Defining critical days as we did above, and picking so many days out of each cycle to be critical, inevitably produces the 77.4% odds, for any kind of cycles, even imaginary ones!