THE NUMBER OF CANCELED BUS TRIPS is rising at the MBTA primarily because of unscheduled absences taken by drivers.

MBTA officials told members of the Fiscal and Management Control Board on Monday that more than 2.5 percent of bus trips were canceled during the last three months of 2017, which was up almost 1.5 percentage points since the start of 2016 and on a par with the snowmageddon months of early 2015.

The officials said the chief cause of the canceled trips is unscheduled absences by drivers. Of the 1,430 drivers needed to provide service each day, the T said 108, or 7.6 percent, typically don’t show up for work due to unforeseen circumstances, which makes planning for the absences difficult and causes about 300 canceled bus trips.

According to T figures, each driver took 3.25 more unscheduled absence days on average in fiscal 2017 than he or she did in fiscal 2016. The biggest share of unscheduled absence days was for leaves under the Family and Medical Leave Act, a federal labor law providing unpaid, job-protected leaves for family and medical reasons. The T figures indicated 787, or 64 percent, of the 1,232 T bus operators eligible for FMLA have been certified to receive it. Most of the FMLA leaves are classified as intermittent, or unscheduled.

Brian Shortsleeve, a director of the control board who focused a lot of his time on unscheduled absences when he served as the T’s chief administrator and acting general manager, was clearly upset with the upward trend in the numbers.

“We owe it to our riders to create an environment where attendance is valued,” he said. “I think we’ve got a lot of work to do in terms of reducing absenteeism when you see that 65 percent of our bus operators are qualified for FMLA.”

James O’Brien, president of the Boston Carmen’s Union, which represents bus drivers, said the T deserves the bulk of the blame for the problem. “Over the years, management has encouraged people to take FMLA. It’s difficult to get them off once they get on,” he said.

O’Brien said the third-party administrator the T hired to track absences is not working out. He also said the shortage of drivers may be worse that the T’s figures indicate because of the churn in the driver workforce that makes it difficult to accurately track the problem.

The MBTA operates 175 bus routes and provides 14,000 trips per weekday spanning 50 communities. The T said 97.5 percent of its trips are provided, but that percentage falls short of the transit agency’s goal of 99.5 percent.

“We passed a service delivery standard of .5 percent [dropped trips]. We’re certainly not hitting it,” said Steven Poftak, a director of the control board. He said the T needs to figure out a way to reach that goal or admit that it isn’t attainable.

When the T is faced with an insufficient number of drivers, it sometimes responds by shifting drivers from one route to another, which results in less service elsewhere on the system.  Other times the trip is simply canceled. The percentage of canceled bus trips hovered around 2.5 percent in early 2015, declined to 1.2 percent in early 2016, and then steadily rose to return to the 2.5 percent level in early 2018. The third party administrator who tracks absences got up and running in the fall of 2016.

The T’s plan to address the problem includes the hiring of 55 new bus drivers at an annual cost of $3.6 million. The transit agency also plans to roll out a “more robust attendance standard” and schedule medical reviews for workers who exhibit a pattern with their absences.