People trying to walk in were turned away after Rench Road was closed to incoming traffic because motorists were parking along Md. 65, also known as Sharpsburg Pike, according to Maryland State Police.
Rench Road was closed to incoming traffic at about 3:15 p.m. to clear it of traffic so it could be turned into two outbound lanes to handle the barrage of traffic exiting after the battle re-enactment, said Robert C. Arch, chairman of the event's logistics committee.
That was part of event organizers' original plan to handle the thousands of cars and not because parking lots were full, Arch said.
Heavy outbound traffic continued throughout the afternoon and evening as parking lots slowly emptied.
"When you've got to get all these people out it's going to take a while. It's just like when you go to see a baseball game, a football game," Arch said.
State tourism officials estimated about 20,000 vehicles parked in spectator lots Saturday, bringing crowd estimates at more than 40,000 spectators and at least 13,000 re-enactors.
Event traffic starting causing delays on Md. 65 and Interstate 70 at about 7 a.m., said Maryland State Police Sgt. L.A. Mastiano.
Eastbound I-70 ramps were backed up continuously until about 5 p.m. while exiting westbound traffic stretched more than a mile back to Antietam Creek, Mastiano said.
Through-traffic wasn't affected, however, he said.
Police were able to keep Md. 65 open to north- and southbound traffic throughout the day with traffic creeping along, Mastiano said.
Traffic was moving normally on Md. 65 by 6:30 p.m., he said, though motorists were still experiencing delays exiting Rench Road.
As of 7:45 p.m., spectator parking lots were only about 75 percent clear of traffic, Mastiano said.
Considering the number of people the event drew, Mastiano said he felt the traffic situation was "pretty good."
No event-related traffic accidents were reported, he said.
Heavy traffic and delays are expected again today, Mastiano said.
"Sometime tomorrow night we'll be able to breathe," he said.