Change in distance traveled, compared with normal, on March 27 in U.S. counties -60% or less +30% or more no data New York, the state hit hardest by the virus, has experienced some of the largest declines in travel Movement fell sharply in California, where a shelter-in-place order is set to last until April 7 Some of the smallest declines are in northern Florida, Georgia and Alabama, where the number of infections is just beginning to climb Change in distance traveled, compared with normal, on March 27 in U.S. counties -60% or less +30% or more no data New York, the state hit hardest by the virus, has experienced some of the largest declines in travel Movement fell sharply in California, where a shelter-in-place order is set to last until April 7 Some of the smallest declines are in northern Florida, Georgia and Alabama, where the number of infections is just beginning to climb Change in distance traveled, compared with normal, on March 27 in U.S. counties -60% or less +30% or more no data Change in distance traveled, March 27 -60% or less no data +30% or more Note: Uses a three-day average.

The coronavirus pandemic has ground America to a halt from coast to coast. But states in the South and Midwest have not reacted the same way or to the same degree, despite warnings from health experts that drastic measures are needed to slow the spread of the virus.

The impact of social distancing and work-from-home measures has been captured in a new way using data on smartphone movements, an analysis of which was provided to the Times Opinion section by Descartes Labs, a geospatial analysis company.

The data shows how travel has plummeted in states hit hard by the virus, like New York, California and Washington.

But other states with fewer confirmed infections have seen smaller declines, presenting challenges for controlling the spread of the virus. Health experts argue that states without many known infections should react as though the virus is already widespread — because it probably is.

“The cases we see today reflect transmission that has happened in the past,” said Ashleigh Tuite, an epidemiologist at the University of Toronto. “We have to be able to anticipate what is to come.”

Travel declined in all states as the number of confirmed cases grew Note: Uses a three-day moving average. States are color-colored using the Cook Partisan Voter Index , which shows how strongly states lean toward Democrats or Republicans compared to the country as a whole. Confirmed cases reflect those identified by The New York Times

To measure how social distancing has changed our behavior, analysts looked at how far people travel each day using smartphone G.P.S. signals. Then they calculated an average distance for each county and state, comparing travel today against what’s considered normal.

Data collected from smartphones can be imprecise, especially in smaller counties where fewer users share their location. Even still, the analysis offers one of few ways to measure how the country is reacting to the crisis in near-real time.

The South and Midwest lag behind Change in distance traveled on March 26 -60% or less +30% or more Wash. Me. Mont. N.D. Vt. Minn. Ore. N.H. Idaho Wis. S.D. Mass. N.Y. R.I. Wyo. Mich. Conn. Iowa Pa. Neb. N.J. Nev. Ohio Ind. Del. Ill. Utah Md. Colo. W.Va. Kan. Calif. Va. Mo. Ky. N.C. Tenn. Ariz. Okla. ARK. N.M. S.C. MISS. ALA. Ga. TEXAS LA. Alaska FLA. HAWAII Change in distance traveled on March 26 -60% or less +30% or more Note: Uses a three-day average.

The smartphone movements reveal one major turning point: the weekend of March 14 and 15, by which point New York, Massachusetts, Illinois and other states had ordered restaurants and schools to close. By March 16, as the number of confirmed cases surpassed 4,500 nationwide, many parts of the country finally saw significant declines in daily travel.

Southern states like Texas, Florida and Louisiana saw drops in mobility around 40 percent at the beginning of last week, but those numbers have leveled off even as the number of confirmed cases continued to climb. (The total number of infections is likely much higher than what’s been confirmed due to a lack of testing.)

It’s also unclear what role statements from some Republican lawmakers and conservative media outlets played in influencing the reaction in many right-leaning states. Early on, some conservatives portrayed the pandemic as a “media-fueled hysteria,” encouraging constituents to continue eating at busy restaurants despite the warnings. President Trump also played down the risks at first, labeling concern over the virus a mere “hoax.”

Republicans have been less concerned about the coronavirus overall, according to a poll by Civiqs. But those numbers are starting to change as infections spread and stock markets tumble.

Even still, some Republican leaders have acted contrary to recommendations by health experts. Mississippi may be one of the most striking examples. Its governor, Tate Reeves, issued an executive order last week that defined nearly every type of business as essential, allowing restaurants and religious institutions to reopen. This caused wide confusion and left mayors across the state scrambling to walk back restrictions already in place.

The governor later clarified the order was not meant to supersede local government decisions. But the damage was done. Jason Shelton, the mayor of Tupelo, had reversed an order closing restaurant dining rooms the weekend prior.

“We don’t have a choice but to go by what the governor says,” Mr. Shelton, a Democrat, told city council members, according to The Daily Journal newspaper.

Mississippi, which had 485 cases on Wednesday, borders hard-hit Louisiana, whose governor issued a statewide “stay at home” order last Sunday. But Mississippi hasn’t budged.

“We’re not going to make rash decisions simply because some other states decide to do things,” Governor Reeves said. He later told viewers in a Facebook Q&A: “Mississippi’s never going to be China. Mississippi’s never going to be North Korea.”

The lesson of the outbreak so far is that control measures often appear drastic before infections rise, but they are necessary to prevent the situation from worsening. States in the South and Midwest, like Mississippi, need to act now, before they start looking like New York, Washington — or, yes, China.

“People sometimes think that you’re overreacting,” said Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, in an interview with CNN. “I like it when people are thinking I’m overreacting because that means we’re doing it just right.”