As the sun sets on this winter solstice, bringing an end to the shortest day of the year, headlights will flicker on in traffic jams across the country, according to estimates from AAA.

Wednesday and Thursday afternoon were expected to see the most crowded roads for this holiday season.

In New York, Los Angeles, San Francisco and Houston, the worst day for travel was Wednesday.

But in other cities — including Washington, D.C, Chicago, Boston, Seattle, Atlanta and Detroit — the real backlog is predicted to kick in on Thursday afternoon. As holiday travelers combine with everyday commuters, drivers can expect trips to take as much as 2 1/2 times longer than usual.

The "worst time for travel" begins at 2:30 p.m. in Boston and will be in full force in all those cities by 4 p.m. You can see more details at AAA.

Overall, AAA estimates that a record 107 million Americans will travel for the holidays, including 97.4 million traveling by car (3 percent more than last year).

For people traveling by plane, Airlines For America predicts that Thursday and Friday will be the two busiest pre-Christmas days, with most return flights scheduled on Dec. 26.

If you're traveling on Christmas Eve or Christmas Day, meanwhile, crowds will be at their lightest, the organization says.

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