Road Atlanta is a great track for spectator photography. You can get close to the track in a few places with only a 4' fence. Turn 10a/b is a favorite spot of mine with plenty of shooting space & low fences on the inside and outside. The area at turns 6/7 is another good spot. On Spectator Hill you can get some good shots of them coming through the "esses." Just north of Spectator Hill, across the service road, there's a hillside where you can get some good back-end shots of cars going up the hill towards turn 6. You can also shoot down into the "esses" from that vantage point too. At turn 1 you can also get pretty close but there's not much time to track cars after they clear the pit exit walls.
The nice thing about those smaller events is less people and it's so easy to get around the place. Last month I was at the historic races and there could not have been more than 100 people there, including the campers. A big difference from Petit Le Mans where you can barely get around.