If circuit breakers have never been used to halt trading, it follows that we have never had the experience of trying to re-start trading either. To an economist such as myself, some of the scariest times during the market crash were those in which trading was not occurring. Our tendency to worry more about stopping trading than re-starting it is mystifying.
(I realize that there has been some discussion about the rules for the resumption of trading but the overwhelming attention has been on the halt.) Recent re-assessments of circuit breakers have focused on increasing the magnitude of price declines necessary to trigger coordinated trading halts.