Automobile and passenger traffic on Steamship Authority ferries plunged into steep decline this month, just as the coronavirus pandemic began its spread into Massachusetts.

Preliminary traffic statistics released by the boat line this week for the month through March 19 show dramatic downward trends in car and foot traffic on the Vineyard route, beginning in the second week of the month.

Over a five-day period from March 15 to March 19, passenger traffic fell 34.6 per cent overall compared with the previous year. Car traffic overall was down 30 per cent. And trucks under 20 feet — a category that includes standard-sized pickups and large SUVs — were down 9.7 per cent overall.

But there were notable differences between excursion and non-excursion vehicles traveling on ferries between Woods Hole and Vineyard Haven, with excursion car traffic falling dramatically, while non-excursion car traffic rose.

It was the only category to see an increase.

Established for year-round Islanders, the excursion program offers deeply discounted round-trip fares for automobile reservations originating on the Island. Non-excursion fares are generally used by visitors and seasonal residents, who also can register for profile numbers for booking automobile reservations. Profile accounts include the state where the car is registered.

Between March 15 and March 19, excursion fare automobile traffic fell 44 per cent while non-excursion automobile traffic was up 10 per cent, preliminary numbers show. The trend was similar for trucks under 20 feet; in that category excursion fare traffic fell 32.4 per cent while non-excursion fare traffic was up 12.8 per cent.

SSA communications director Sean Driscoll said the tide changed quickly and suddenly.

“It really is that March is the tale of two months,” he said, speaking to the Gazette by phone. “Things were good through March 14 . . . January and February were pretty strong because the weather had been good,” he said. “And then the other March came in, and once that slip began there was no going back from it. I don’t know if we’re at the bottom of it yet.”

Prepared as of March 20, the statistics released this week are preliminary and still subject to audit. But they show a snapshot in time on the boat line, as the pandemic began to unfold.

Until March 7, traffic on the Vineyard route looked relatively normal, with passenger traffic up 5.7 per cent for the month and 6.6 per cent for the year. A total of 233,143 passengers had been carried for the year, 14,418 more than during the same period in 2019. Through March 15, ferries on the Vineyard route carried a total of 13,637 vehicles (cars and trucks under 20 feet).

Automobile traffic as of March 7 was down 1.7 per cent for the month but still up 3.4 per cent for the year. But excursion fare traffic had begun to fall off, dropping 7.5 per cent for the month. By contrast, non-excursion traffic was up 9.7 per cent month to date. Non-excursion trucks under 20 feet were also up 5.3 per cent for the month at that point, while traffic for excursion trucks under 20 feet fell 1.6 per cent.

Day by day, beginning March 15, the numbers went into steep decline — except in the non-excursion automobile categories, where the numbers climbed slightly.

Year to date as of March 19, the boat line was still in the black, with overall passenger traffic up 2.8 per cent, overall car traffic up .9 per cent, and overall truck traffic up 1.6 per cent.

Chartered to provide dependable year-round service to the two Islands, the boat line typically loses money in the winter months and makes up for it in the busy summer months. Ordinarily March would mark the beginning of an upward trend in traffic and revenues, with the Easter season and April school vacation ahead, followed by Memorial Day and the start of summer.

A reduced ferry schedule went into effect this week, with fewer trips. The service cuts were accompanied by about 65 employee layoffs, Mr. Driscoll said.

And an advisory has been posted on the SSA website from both Island hospitals urging people with second homes to stay away.

In an attempt to track available data about second home owners seeking refuge on the Vineyard during the pandemic, the SSA also released traffic statistics by license plate from March 1 to March 15. The statistics show modest increases in out-of-state travel to the Island.

During that period cars with a Massachusetts plate saw a net increase of 264 vehicles. Customers with a New York/New Jersey plate saw a net increase of 102 vehicles. Customers with New England state plates other than Massachusetts saw a net decrease of 21 vehicles.

“It does show there is a big increase in Massachusetts plates coming over, and there is a bump in New York and New Jersey,” Mr. Driscoll said. “But it’s not overwhelming. In that time period we carried 13,000 cars — a net flow of 100 it doesn’t seem huge to me.”

Meanwhile, Mr. Driscoll said while no hard numbers are available yet, ferry traffic across the board has continued its freefall since March 19.

“You could hold ballroom dancing classes on our freight decks this week,” he said. “We are still bringing trucks over in the morning, so at that time it is more crowded. But that’s about it.”