Decades of little-noticed efforts to build guardrails for wild market moves have gotten a rare workout in recent days as circuit breakers have repeatedly halted trading in all U.S. stocks, most recently on Monday morning.

Have they been effective? The jury is out.

On March 9, on Thursday and again on Monday, a 7% plunge in the S&P 500 during the opening minutes of trading forced 15-minute, marketwide halts—the first time since 1997 that the mechanism has been activated. None of the circuit-breakers prevented stocks from tumbling further: On March 9 the S&P closed 7.6% lower, on Thursday it fell 9.5%, and on Monday it slid 12% in its steepest one-day drop since “Black Monday” in 1987.

It’s impossible to know, of course, how markets would have acted if the halts hadn’t happened. But some traders complained they did little good.

“It’s just spooking the market,” said Dennis Dick, a trader at Bright Trading LLC. “I think they’re more disruptive than helpful.”