The tarmac delay rule the U.S. Department of Transportation (DOT) implemented in 2010 has significantly reduced long tarmac waits, but it has served to increase overall delays that passengers experience, mainly due to flight cancellations, a new study finds. The study authors at Dartmouth College and the Massachusetts Institute of Technology recommend modifying the current rule to increase the allowed tarmac hold time from three to 3.5 hours.

Responding to a series of incidents in which airline passengers were stranded on aircraft for lengthy periods, the DOT announced the tarmac delay rule in December 2009, and it went into effect in April 2010. The rule prohibits U.S. airlines conducting domestic flights from allowing an aircraft to remain on the tarmac for more than three hours without deplaning passengers. International carriers operating to or from the U.S. are allowed four hours. Exceptions are made if the pilot determines that deplaning passengers would be a safety risk, or if ATC decides that doing so would disrupt airport operations. Violators can be fined up to $27,500 per passenger.

The Dartmouth-MIT study, “Tarmac Delay Policies: A Passenger-Centric Analysis,” determined that flights with three hours or longer of tarmac time decreased by 98 percent from 604 in 2009 before the rule took effect to just 11 in 2013. The rule “seems effective” in preventing lengthy tarmac delays, the authors acknowledge. But they argue that disruptions from flight cancellations, diversions and misconnections have increased delay times, especially for passengers scheduled to travel on flights that are at risk of long tarmac delays.

While other studies have concluded that the tarmac rule has increased the probability that flights with long taxi-out times will be cancelled, the Dartmouth-MIT study is the first to analyze the rule’s effectiveness from the passenger perspective, the authors contend. “Flight delay alone can considerably underrepresent the delay to passengers,” they state. “For example, as a result of a two-hour flight delay, a passenger on this delayed flight with a one-hour connection time misses his/her connecting flight leg, and has to wait, say three more hours, for the next flight with an available seat to his/her final destination. This situation results in a passenger delay of four hours, double the two-hour flight delay.”

Using an algorithm to estimate passenger delay, the researchers quantified delays to passengers in the peak year of 2007, when there were 1,654 instances of taxi-out times of three hours or longer, and compared those to estimated delays for hypothetical scenarios with the tarmac rule in effect. “Our main result is that, while the three-hour tarmac delay rule (in its current form) effectively decreases tarmac delays, especially the extremely long tarmac delays, each passenger-minute of tarmac time saving is achieved at the cost of an increase of approximately three passenger-minutes in total passenger delays,” the study states.

A “better balance” between conflicting objectives of reducing the frequency of long tarmac delays and reducing overall passenger delays could be achieved by modifying the current rule, according to the study. The authors recommend increasing the tarmac time limit to 3.5 hours and applying the rule to flights with departure times before 5 p.m. They also recommend that the clock should start when an aircraft begins returning to the gate, rather than when passengers are allowed to deplane.