head gaskets

What is the chance of seeing two cars blow a head gasket in the time of one morning’s commute?
(Considering the distance of mine, it may be a little more probable)
But I did see it happen twice this morning. Both with rolling bluish-white smoke billowing out from under the car, one with water splashing everywhere. And both drivers seem to be completely unaware of what was happening! Both were driving along at 70-80 MPH, and just smiling as I went by (not that I was driving faster than 70-80). I guess the smokescreen behind them, the smell of hot, oily anti-freeze, the loss in power, and the distinctive “ch-ch-ch-ch” didn’t tip them off that there was a problem.