(Washington, DC -- David Schultz, WAMU) Data released today by the U.S. Department of Transportation shows a new rule designed to prevent airlines from delaying their passengers on airport tarmacs may be working.

Only three flights in July of this year experienced a tarmac delay of three hours or more, the DOT says. That's compared with 161 flights with three hour tarmac delays in July of 2009.

DOT enacted its new tarmac delay rule in April. The rule was designed to combat the growing trend of passengers stranded on grounded planes for hours. Under the new rule, airlines are prohibited from keeping passengers on a grounded plane for more than three hours, unless there are mitigating safety or security factors.

Some airline industry analysts had worried this rule would prompt the airlines to simply cancel flights if they felt they couldn't make that three hour window. But, according to DOT data, that hasn't happened.

Large airline carriers only cancelled 1.4 percent of their domestic flights in July, the DOT says, only slightly up from the same time last year - and slightly down from the month before.

For more info, see this report.