Circuit breakers have never actually been triggered, in contrast to some of the so-called “speed bumps” which affect particular trading strategies and are now tripped routinely. (In contrast to circuit breakers, which are coordinated across the equity and derivative markets, speed bumps are trading restrictions that have been put in place by individual market places.)
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.