Between May 2010 and April 2011, the first 12 months after the time limit was in effect, airlines reported 20 tarmac delays of more than three hours, none of which was more than four hours long.
In contrast, during the 12 months before the rule took effect, airlines had 693 tarmac delays of more than three hours, and 105 of the delays were longer than four hours.
looks to me the rule is very doable