Trigger points of 10 percent and 20 percent would make sense. Second, they should be coordinated between the stock markets and the stock index futures markets, so that no danger exists that one set of markets will be open and the other closed. The reset should be automatic, just as the circuit breakers themselves are automatic. Nobody would be required to intervene in the resetting, so no question would arise that the reset indicates that the market has become more or less volatile. The trigger points are simply being automatically reset to take account of changes in the market level.
Circuit breakers have a useful role.
They should be called into play when markets are racing ahead of people's ability to deal in them, when markets have become disorderly, when prices are not being set by new information, but not when markets are functioning in an orderly way. In those cases markets should be allowed to trade and seek their own level. The stock people and the academics who were on the Brady Commission did not understand futures markets and derivatives markets. As Leo Melamed has pointed out, futures traders settle every day. They know who is solvent and who is not solvent, but the stock people do not work that way.