If history holds true, most of those motorists will start their Thanksgiving trips today, officials said.
"The Wednesday before Thanksgiving is our heaviest day of the year," said Pennsylvania Turnpike Commission spokeswoman Kathy Liebler, who said turnpike officials were bracing for an estimated 1.8 million motorists over the weekend.
The holiday weekend officially started at 12:01 a.m. today, Liebler said.
But traffic was starting to pick up on Tuesday, she said.
Based on past experience, Liebler said she expected traffic volume would keep building throughout today, then taper off early Thursday.
Because of return traffic, Sunday is generally the turnpike's second busiest day of the year, she said.
To keep traffic moving as smoothly as possible, turnpike officials ask their contractors to limit construction over the holiday weekend so at least two lanes are open in each direction, Liebler said.
Toll booths are at full staff over the weekend, she said.
The really heavy traffic will start late this afternoon as people getting off work hit the road and it won't let up until a little after midnight, said West Virginia State Police Sgt. J.A. Humphrey.
Thanksgiving Day traffic is usually light to moderate, "almost like a Sunday morning," said Humphrey, of the Martinsburg, W.Va., barrack.
It picks up again on Friday with the shopping traffic, then gets very heavy Saturday and Sunday as people return home from their trips, Humphrey said.
Washington County roads, especially Interstates 70 and 81, will start getting congested around 4 or 5 p.m. today and probably will stay that way until midnight or 1 a.m., said Maryland State Police Sgt. Terry Hill.
Traffic will be heavier than normal until about 2 or 3 a.m. Thursday, but then will get "pretty quiet" until people start heading home Thursday evening, said Hill, of the Hagerstown barrack.
The bulk of holiday travelers won't return home until Sunday, when police expect the flow to be as heavy as that on Wednesday night, he said.