As it has for more than 40 years, metro Atlanta will kick off Independence Day on Thursday with The Atlanta Journal-Constitution Peachtree Road Race. The race, which is the largest in the United States since the bombing of April’s Boston Marathon, will take place under heavy security. Here are five things you should know if you plan to run or watch.

1. Watch the flags. Colored flags along the course's 6.2 miles will inform runners and spectators of the threat level during the event. Green flags indicate optimal conditions, while a yellow flag means to be on alert. A red flag is more serious, as it means conditions are potentially dangerous. A black flag indicates extreme danger and a halt to the event.

2. Baggage check. The heightened security will mean more controlled movement, additional fencing at the start and end of the race and security checks of backpacks and bags. Anyone trying to get to the Meadow at Piedmont Park, where an estimated 60,000 runners will cross the finish line, will have their bags and backpacks checked. Race organizers recommend leaving bags at home if possible.

3. Take MARTA. The Atlanta Track Club strongly recommends runners and spectators take MARTA to avert traffic and parking problems. The transit system will begin running at 5 a.m. on race day. The track club is encouraging runners to use the Lenox station for the start and the Midtown or Art Center station for the finish.

4. Smile, you're on camera. The Atlanta Police Department says 1,400 cameras will be monitoring the course. Atlanta Police Chief George Turner says the surveillance will allow for quicker responses in case of an emergency.

5. See something, say something. MARTA wants riders to say something if they see something suspicious, from unattended packages to people behaving strangely. MARTA has developed an application available on Apple and Android smartphones to report suspicious behavior. The same philosophy extends to the race, where Atlanta Police want race participants to dial 911 if they see something that should be checked out.