Ever wonder how a pilot knows to stay clear of an airshow? According to Federal Aviation Regulations 91.03 "Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight." Therefore it is the pilot's responsibility to determine whether there are any hazards along the intended route of flight.
A pilot can take advantage of many online services to fulfill this requirement or make a call to flight service briefer. Before each of my flights I contact Flight Service for a last minute check on the weather and also to double check for TFR (Temporary Flight Restrictions) or NOTAMs (Notice to Airmen). If I had been intending to fly along the lakefront I would have learned during this process that there was a Notice to Airmen restricting flight within five miles of the airshow center.
In the example of the Chicago Air & Water Show they communicate the location by using the show centers location in regards to the Chicago O'Hare VOR. For example the Airspace is defined as having a show center located at 13.6 nautical miles out from the Chicago O'Hare VOR on the 106° radial. The flight restriction is then for a radius of 5 nautical miles from that point from the surface to 16,000 feet.
You can view a sample of the Notice on the FAA website.
