New York City’s transportation system typically carries eight million riders each weekday. But the sudden and steep drops in ridership have severely strained the authority’s operating budget, about half of which comes from fares and tolls.

On Monday, ridership had dropped 70 percent on buses compared to the same day last year. Ridership on Metro-North dropped 94 percent and on the Long Island Rail Road fell 76 percent at the end of last week compared with the same period last year, officials said.

“It was just a matter of time for this logical next step. With increasing worker shortages and decreasing ridership, we appreciate that the M.T.A. will keep a good level of service to get front line workers where they need to go,” said Lisa Daglian, executive director of the Permanent Citizens Advisory Committee to the M.T.A., a watchdog group.

The M.T.A. expects revenue losses of roughly $3.7 billion if ridership trends continue for the next several months, and projects that coronavirus-related expenses, like disinfecting its equipment, will reach around $300 million.

These estimates do not take into account what is sure to be a major drop in the roughly $6 billion the authority receives from dedicated state and local taxes.

The reduction in service allows the M.T.A. to lower its operating costs and help stave off what the authority has called a “financial calamity.”

But health professionals have raised concerns that running fewer trains will lead to more crowding on trains that remain in service, increasing the risk that passengers — many of whom are essential workers like doctors and nurses — are exposed to the virus during their commute.