The Department of Transportation warned drivers on Monday not to get distracted by a rare, once-in-a-lifetime total solar eclipse that will be visible from more than a dozen states on August 21.

The department said the complete eclipse will be visible for millions of people coast to coast, "from Oregon to South Carolina." And it will happen during the middle of the day, "when millions are on American roads, potentially causing one of the largest driver distractions in years."

The Federal Highway Administration urged drivers not to pull over to look at the eclipse in unsafe spots, and said they should be mindful of pedestrians and cyclists who might also be looking up.

"Because the totality will also cross countless smaller state routes, county roads and city streets, it is not known exactly how many people will be watching the skies that day from their cars — but we know it could be several million drivers," it said.

"It will be short-lived, but we expect more traffic congestion that day," it added. "As a result, FHWA urges travelers to plan their road trips in advance by visiting the 14 state DOT websites for real-time traveler information."

The department has a website on the upcoming eclipse that includes transportation information, safety tips and information from the 14 states that will see a total eclipse.

The department said the only way to look at an eclipse safely is through "special-purpose solar filters, such as eclipse glasses or handheld solar viewers."