Hi Dave:
For perspective, I've been to about 40 local pattern contests over 5 seasons, as well as 4 Pattern NATS. There have been three midairs that I know about in those local contests, one that caused minor damage to both models, one with a total loss of both, and one with a loss of one and major damage to the other. At the '04 NATS, there were 2 mid-airs I can remember from the FAI line. There may have been others over '05 through '07, but I don't recall any. There were none in '08. Particularly with the NATS, where two of the three flying sites at Muncie have the same class going simultaneously on both flight lines, and with a higher percentage of pilots of pilots who hold a consistent line than at local contests, there is a low number of midairs overall. This years there were 128 pilots competing at the NATS; typical numbers are 110-120 each year.

Since there's virtually no cross box stuff in Pattern, except the occasional Humpty with options, you can pretty much pick a line and stay in or out from where the other guy is most of the time. Not a perfect solution, but it helps some.