Organizers estimated 7,000 spectators attended the event along with the hundreds of school children bused in that morning for free tours.
That traffic, mixed with a constant flow of re-enactors coming in throughout the day, caused delays getting into the parking lots that backed up north- and southbound Sharpsburg Pike, officials said.
At 1 p.m., Maryland State Police issued a news release requesting motorists avoid the area of Sharpsburg Pike and Interstate 70 and warning of traffic delays there.
Shortly before 2 p.m., the southbound line to get on to Rench Road started at Col. Henry K. Douglas Drive while northbound traffic stretched backed to Wagaman Road.
Even at the height of the crunch, troopers were able to keep Interstate 70 moving, said Sgt. Denton Lowery.
However, there were backups in the slow lanes due to delays at the ramps, Lowery said.
A major traffic jam occurred on Rench Road at about 2 p.m., when school buses carrying students started to leave.
The buses had to compete for space on the two-lane Rench Road with hay wagons shuttling spectators and vehicles going in and out of the site.
Crowding the road even more were thousands of re-enactors marching to fight in a field near the site entrance.
"It's been a hectic couple of hours," said Spencer Brown, a volunteer from Manassas, Va., who was directing traffic at a parking lot entrance.
Jerry and Tracy White of Detroit said it took them two hours to park, but they didn't seem to mind.
"We figured it was a big event so we weren't very surprised it was so crowded," Tracy White said.
Gene Renner of Hagerstown and Lisa Hendrick of Martinsburg, W.Va., tried to sidestep the traffic by parking their motorcycle near St. James School and walking about 30 minutes.
"We jogged part of the way," Hendrick said.
Organizers were going over traffic and parking plans Friday afternoon in an effort to minimize problems today, said Nancy Jones, a traffic engineer with the Maryland State Highway Administration in Hagerstown.
Changes in the parking plan and a slow-down in re-enactor traffic should help, Jones said.
Rench Road was still backed up to Sharpsburg Pike as of late Friday as re-enactors continued to move in, said Doug DeHaven, deputy fire chief of Halfway Volunteer Fire Company.
Staff writer Guy Fletcher contributed to this story.