Nearly 860,000 additional travelers flocked to parts of Maryland and Virginia over the weekend as the states began to reopen Friday, according to researchers tracking smartphone data. Many were from the Washington suburbs, which remained shut down because of their significantly higher coronavirus caseloads, the data shows.

The 18 percent jump in travel from previous weekends brought a total 5.83 million trips to those areas between Friday and Sunday, according to an analysis by University of Maryland researchers.

Virginia Gov. Ralph Northam (D) and Maryland Gov. Larry Hogan (R) lifted shutdown orders on some businesses Friday but allowed the hard-hit Washington region to remain closed at the request of local leaders.

The increase in travel, including from coronavirus hot spots, underscores concerns among public health experts that piecemeal lifting of restrictions increases the chances of the virus spreading. They say they’re particularly concerned about rural areas, where there have been fewer cases, as people travel from closed areas to those that have reopened.

“It’s a really bad scenario,” said Lei Zhang, lead researcher on the University of Maryland project.

By reopening some parts of states considered at lower risk ahead of coronavirus hot spots, Zhang said, “It actively encour­ages people to travel from high-risk areas to these otherwise safer, more rural areas without many cases. That’s certainly not a good trend.”

He said the findings also show the growing difficulty of containing the novel coronavirus in a highly mobile country, where people travel frequently between counties and states, both for work and leisure. He noted that the increase in travel started nationwide several weeks ago, well before the start of the busy summer season.

Maryland counties with beaches, boating and outdoor recreation saw the biggest jumps, including Cecil County on the Chesapeake Bay, Worcester County (home to Ocean City) and Garrett County (home to Deep Creek Lake). In Virginia, the top destinations were Bristol on the Tennessee border, Bland County on the West Virginia border and Emporia, close to the North Carolina line.

The findings were similar to a 13 percent weekly jump that researchers saw in cross-state travel to Georgia after it became one of the first states to reopen most businesses in late April. Most of those visitors came from surrounding states that remained shut down at the time. The 18 percent jump in visitors to the Delmarva region is probably higher because it was concentrated over a weekend and occurred in a densely populated area, Zhang said.

Most of the weekend trips did not entail commuting, he said.

Public health experts said the University of Maryland findings, derived from location data on smartphone apps, don’t reveal how much people are social distancing, wearing masks or taking other precautions as they travel.

Even so, they say, traveling generally increases personal interactions, making it easier to transmit the virus. It also makes contact tracing more difficult, they say, because people who later discover they’re infected are less likely to know with whom they interacted.

Meagan Fitzpatrick, an assistant professor at the University of Maryland School of Medicine who reviewed the findings, said she wasn’t surprised, based on Georgia’s experience.

Even so, she said, the significant increase in travel shows that counties need to go beyond considering infections among residents when assessing their readiness to reopen.

“It shows the more accurate risk assessments take into account infections from areas your county will draw from,” Fitzpatrick said. “What are they bringing with them?”

Fitzpatrick said researchers won’t know for two to three weeks whether the increase in travel leads to more infections. That’s because it can take a while to see trends rather than daily blips due to when and how test results, hospitalizations and deaths are reported.

In Georgia, for example, an analysis of state data by the Atlanta Journal-Constitution found that it’s too early to know the health impacts of the state’s April 24 reopening. Cases of the novel coronavirus have held steady or dropped slightly in the past two weeks, the paper said. However, its analysis found that recent data is too preliminary to show clear trends since the reopening.

Zhang, director of the Maryland Transportation Institute, said stepped-up travel in the Delmarva region matches national trends. Nationwide, about 47 million people crossed state lines Friday. That was more than twice as many as in mid-April and climbing back toward the pre-pandemic daily average of 55 million, even as government officials in many parts of the country continue to ask residents to stay home as much as possible to slow the virus’s spread.

“How quickly these out-of-state trips are coming back is really shocking,” he said.

Maryland and Virginia’s reopenings coincided with a sunny, warm weekend. There were reports of throngs of people on the Ocean City boardwalk, with many not wearing masks or practicing social distancing. Recreational trails also attracted crowds of cyclists and hikers.

Ocean City Mayor Rick Meehan said he was concerned about visitors bringing the virus from places that remain under stay-at-home orders.

“They’re in place for a reason,” Meehan said. “We’ve never encouraged people to violate those orders. . . . If people are in a hot spot, they shouldn’t be traveling. They should be following those orders.”

Zhang said researchers estimate that 40 percent of the new weekend trips in Maryland and Virginia were related to busi­nesses reopening. The other 60 percent, he said, followed trends seen over the past several weeks attributed to “quarantine fatigue,” as people sick of mostly staying home have begun to venture out.

He noted that many of the Maryland businesses allowed to reopen weren’t the kind that attract weekend visitors. Travelers to Ocean City, for example, still had to line up outside restaurants for takeout.

“I think people just want to be outdoors in places that don’t feel like home,” Zhang said.

Luisa Franzini, chair of the health policy and management department at the University of Maryland School of Public Health, said she also suspects much of the travel stems from a growing restlessness rather than a desire to frequent newly opened businesses.

She said people appear to be following their own feelings of risk rather than government decrees, particularly those seen more as recommendations because they aren’t strictly enforced.

“Maybe people are just less concerned or less scared,” Franzini said. “They’re just getting out more. Certainly there’s an effect of reopening. But it’s not like you turn a switch on and people travel. It’s been an ongoing trend.”

The parts of Maryland that began to reopen saw the biggest increase — nearly 570,000 new weekend visitors — probably because it is closer to population-rich states such as Pennsylvania and New Jersey, Zhang said.

Hogan allowed hair salons, pet groomers, carwashes, manufacturers and certain retailers to reopen, with restrictions, in 16 counties. Gyms remain closed statewide, as well as eat-in restaurant dining.

However, local officials in Montgomery, Prince George’s and Charles counties, as well as the city of Baltimore, kept their shutdown orders in place, saying they still had too many coronavirus infections to reopen safely. Howard, Frederick, Anne Arundel and Baltimore counties allowed some reopenings but with tighter restrictions than the state required.

About 40 percent of new travelers to reopened parts of Maryland came from Washington suburbs that remain shut down, Zhang said. About 7 percent came from the District, where most unessential businesses remain closed. About 47 percent came from other states, including Pennsylvania, Delaware and West Virginia.

In response to questions about reopening some parts of the state before others, Hogan spokesman Michael Ricci said, “We continue to follow the road map to recovery we developed with input and guidance from our public health experts, and as the governor has said, each of us has an obligation to continue to exercise responsibility for ourselves, our families, and our fellow Marylanders.”

Northam has allowed restaurants to resume some outdoor seating and some retailers, barber shops and hair and nail salons to reopen with restrictions. He excluded Northern Virginia, Richmond and Accomack County, on the state’s Eastern Shore, at the request of local officials due to their higher number of infections.

About one-third of the travelers to Virginia came from areas of the state still shut down, while most of the other two-thirds came from states other than Maryland, according to the data.

Northam spokeswoman Alena Yarmosky didn’t address public health experts’ concerns about states reopening some parts and not others. However, she said, “Northern Virginia remains under a stay-at-home order to protect residents. While it is difficult to stay inside, these emergency rules are in place to protect individuals, the community, and the Commonwealth during this pandemic.”

Those residents, “must continue to stay home,” she said in an email.

Zhang said the findings show the need to closely monitor how travel from coronavirus hot spots might contribute to outbreaks elsewhere. For example, he said, Baltimore County had more infections in mid-April, when it had the most travelers between Maryland and the New York area. Prince George’s, which has been hit hard, also has had a particularly high number of people arriving from out of state, he said.

Fitzpatrick, the medical school professor, said she understands the growing desire “to get back to normalcy.” But she said residents in hot spots need to refrain from traveling. Their continued shutdown orders indicate not only their greater risk of infection, she said, but also their higher likelihood of spreading it elsewhere.

“We need a lot more clarity about what shutdowns mean for people still under lockdown,” she said. “If that clarity is not going to come, then I think we need to revisit the wisdom of reopening piecemeal.”