Staffing shortages will force MARTA to reduce the frequency of most of its bus routes beginning next month.

The agency — which has seen ridership drop to about half what it was before the coronavirus pandemic — plans to move 96 of its 113 routes to a Saturday schedule beginning Dec. 18. That means buses will run less frequently, even on weekdays.

Eleven of MARTA’s most-used routes will maintain regular schedules, and six routes that don’t operate on Saturday will also keep a regular weekday schedule.

No routes will be eliminated, and there will be no changes to rail service.

“This is not something we want to do,” CEO Jeffrey Parker told the MARTA board of directors Thursday. “This is something we believe is a rational and appropriate thing we want to do under the circumstances.”

MARTA’s budget includes funding for 1,366 bus drivers. But the number of drivers is down to 1,179 — 14% below full strength.

Deputy General Manager Collie Greenwood said employees have left for a variety of reasons, but the coronavirus pandemic is a big factor — so far, 921 MARTA employees have tested positive for COVID-19. Greenwood said absenteeism also has taken a toll on bus staffing.

An additional 60 drivers have ignored MARTA’s recent requirement to be vaccinated for COVID-19 or undergo weekly tests, and they now face termination. If MARTA fires those drivers, its bus driver workforce will be down 18% from full strength.

As staffing has fallen, MARTA has canceled routes — sometimes giving customers little notice. Greenwood said reducing the bus service schedule will allow MARTA to better staff its routes and give customers more predictable service.

“We felt that the alternative was to continue to post the same schedules and have the customers kind of guess as to what the level of service would be tomorrow,” Greenwood told the board.

MARTA officials aren’t sure how long the service reductions will last. The agency plans to restore service gradually as it hires more drivers.

Greenwood said MARTA has stepped up recruiting efforts — including reaching out to retired drivers — to address the staffing shortage.