The Massachusetts Bay Transportation Authority canceled more bus rides than anticipated in 2017, which the agency says is due to an uptick in driver absenteeism. About 300 of 14,000 bus rides were canceled each day last year, with an even higher rate in the last three months of the year.

The T is planning to hire 55 more drivers in the next year and to change its attendance policies in an effort to cut down on dropped trips.

So which routes are canceled the most? The T does not yet have line-by-line information for 2017, but data from 2016 show which routes were most commonly affected that year.

Here were the 15 bus routes with the highest rates of canceled trips in 2016:

77 (Arlington to Cambridge) — 6.2 percent

111 (Chelsea to Downtown Boston) — 5.2 percent

31 (Mattapan to Forest Hills) — 4.1 percent

73 (Watertown to Cambridge) — 3.8 percent

32 (Hyde Park to Forest Hills) — 3.8 percent

57 (Watertown to Downtown Boston) — 3.4 percent

79 (Arlington to Cambridge) — 3.2 percent

23 (Ashmont to Ruggles) — 2.8 percent

1 (Cambridge to Roxbury) — 2.5 percent

39 (Forest Hills to Back Bay) — 2.5 percent

71 (Watertown to Cambridge) — 2.5 percent

28 (Mattapan to Ruggles) — 2.3 percent

66 (Cambridge to Roxbury) — 2.3 percent

15 (Dorchester to Ruggles) — 2.3 percent

502 (Watertown to Back Bay) — 2.2 percent

The MBTA says nearly all of these routes are classified as “key bus routes,’’ which run with higher frequencies throughout the day. Therefore, the agency says, it borrows more often from these lines to fill gaps on lines that run less frequently. (The 31, 79, and 502 are not listed as key routes.)

Cancellations for the 77 and 111 buses are particularly high, the T says, because they are the only key routes that run out of their respective garages. That means they are relied upon to fill the gaps in other lines even more than other key routes.

Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter at @adamtvaccaro.