Editor's note: This story was originally posted on June 19 and updated with new resource links on Aug. 19.

With just days to go before the Aug. 21 total solar eclipse, one big question is: Just how many people intend to travel into the path of totality, which stretches from Oregon to South Carolina?

"Imagine 20 Woodstock festivals occurring simultaneously across the nation," said Michael Zeiler, an eclipse cartographer who estimates conservatively that between 1.85 million and 7.4 million people may commute into the path. But unlike a concert, there are no ticket sales for the eclipse, so no one has a definitive count of how many people will attend. The only thing experts can do is speculate.

Unfortunately, there is no recent precedent on which to base that speculation. It has been nearly 40 years since the last total solar eclipse passed over any part of the contiguous United States. On that occasion — Feb. 26, 1979 — the moon's umbral shadow passed over just five states in the Pacific Northwest and Northern Plains. [How to View a Solar Eclipse Without Damaging Your Eyes]

Solar Eclipse 2017: Traffic and Weather Forecasts for States in Totality

Here's a Timeline of When the 2017 Solar Eclipse Begins and Ends

Here Are the Best Maps of the 2017 Solar Eclipse

This year, the lunar shadow will touch portions of 14 states on its coast-to-coast jaunt across the United States. There is only one other coast-to-coast eclipse to compare it to, but that occurred nearly 100 years ago, on June 8, 1918, more than five months before the end of World War I. Needless to say, it was a whole different world back then!

What can be said with certainty is that 12 million people already live within the totality path, which averages 68 miles (109 kilometers) in width and stretches for 2,500 miles (4,000 km).

But there are also about 200 million people — nearly two-thirds of the total population of the U.S. — who live within a one day's drive (roughly 500 miles, or 800 km) from the path.

But just how many of those 200 million actually plan to travel to see the eclipse?

A commuter nightmare?

Nobody really knows for sure, although the potential certainly exists that the weekend leading up to Monday, Aug. 21, might see more travelers on the roads, on the rails and in the air than at any other time in 2017. And this includes the Thanksgiving, Christmas and New Year's holidays.

The last two weeks in August are also when most people schedule their annual summer vacations.

Earlier this year, at the 26th annual Northeast Astronomy Forum (NEAF) in Rockland County, New York, I jokingly said that depending on weather conditions along the eclipse path, people jockeying into position to view the eclipse "Might make local roadways resemble Brownian motion." (This physical phenomenon describes things like cream moving through coffee, in which many small solid objects occupy a space together, all of them constantly moving in random directions.)

This graphic by eclipse cartographer Michael Zeiler shows the path of the Aug. 21 total solar eclipse, as well as major routes into the path from highly populated areas. (Image credit: Michael Zeiler/GreatAmericanEclipse.com, used with permission)

But while it's a good bet that (thankfully) not all of those 200 million potential eclipse watchers will make the attempt to see the eclipse, there might be unprecedented numbers of people who indeed will try to see this amazing spectacle for themselves.

My personal feeling is that many will make a last-minute decision to jump in their cars and travel to see the total eclipse, that all the publicity and urging leading up to this "once in a lifetime" event will make people want to see it. They might also use it as an excuse to take a Monday off from work.

A good example might be the Atlanta metropolitan area, home to over 5,700,000 people and the ninth largest metropolitan area in the United States.Atlanta is just outside of the totality path and will witness a 97 percent partial eclipse. If anyone in the Atlanta metro area wants to experience the total eclipse, all they would need to do is travel about 80 miles (130 km) to the northeast on Interstate 85, the arterial road of the Southeast.

Normally, such a trip would take about 90 minutes. But while most who live in Atlanta might initially say they're not planning to travel into the totality zone, many could very well change their minds the night before, or even the morning of the eclipse, and head out of town to get a look at the much-ballyhooed big sky show.

It is, after all, human nature to make a last-minute change of plans.

If that happens, then on that Monday morning, Interstate 85 could turn into the world's longest parking lot and a 1.5-hour drive might take twice as long — or maybe even longer.

But transportation agencies are already gearing up for just such a possibility.

State and local departments of transportation planning for the eclipse



The Office of Operations of the U.S. Department of Transportation (USDOT) has issued a fact sheet regarding the effects that the impending eclipse may have on roadways nationwide, especially in the 14 states that the moon's dark shadow will touch.

Think of the eclipse, the USDOT advised, as:

A planned special event for which there has been no recent precedent in the United States

A planned special event that is a feat of nature and not human-made

A planned special event with many different events across the country

An act of nature that is not a disaster

"Travelers should be at their observation location a minimum of a couple hours before totality," advised the Federal Highway Administration. "The role of state and local DOTs may include instituting roadblocks or other measures to keep people from making illegal turns as they drive around looking for 'the perfect spot' as eclipse totality nears."

And there is also anticipation of a mass exodus: "Departures will be more compressed as there is no reason to remain after the period of totality has passed."

Editor's Note: Travelers should also keep in mind the potential dangers of being stuck on a freeway in late August, such as heatstroke. Members of the AAS eclipse task force said peopleshould keep in mind where they will have access to water, food, bathroom facilities and gasoline, especially when traveling with young children or elderly persons. Visit our sister-site, Active Junky, to learn the basics of assembling an emergency supply kit.

Population statistics

Zeiler, who created the fantastic website GreatAmericanEclipse.com, has taken the time to specifically look into the potential number of people who will travel into the total eclipse path. He has done a statistical study of the numbers of people that he believes will make an effort to see the total eclipse. On the positive side, Zeiler points out that there should be plenty of room inside the predicted corridor of darkness for these visitors — provided people distribute themselves well.

"The problem," notes Zeiler, "is that these millions of Americans will produce predictable traffic congestion. …. Large numbers of visitors will overwhelm lodging and other resources in the path of totality. There is a real danger during the 2 minutes of totality that traffic still on the road will pull over at unsafe locations with distracted drivers behind them."

As for how many people will migrate into the totality path, Zeiler has calculated that an average person living 200 miles away from the totality path has a high probability of 2 percent to drive into the path and a low probability of 0.5 percent. For people living 400 miles (640 km) away, Zeiler cut these numbers in half. For those living 800 miles (1,300 km) away, he halved these numbers again. His subjective sum estimate from this analysis is that between 1.85 million and 7.4 million people will visit the path of totality on eclipse day.

Important disclaimers!

But Zeiler added that his numbers could be altered by a number of other factors, such as the heavy promotion of the eclipse on social media sites (including Space.com's accounts), the ever-changing weather conditions along the eclipse path, and the possibility of western wildfires producing widespread smoke that could hinder visibility and force eclipse viewers to head farther east. There are also major regional events that may prove attractive. For instance, Zeiler said, "a very large eclipse-viewing event at St. Joseph, Missouri, will draw visitors from Kansas, Nebraska and [other] nearby states."

And lastly, Zeiler pointed out that his calculations do not take into account potential viewers from other countries, most notably Canada and Mexico, but also Europe, Asia or other international regions. Needless to say, taking all of these things into account, his estimate of between 1.85 million and 7.4 million people traveling to view the eclipse could be conservative to say the least!

The full details of Zeiler's comprehensive study, which also contains an eclipse web app for real-time traffic data, can be accessed by visiting his website.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Fios1 News in Rye Brook, NY.

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