Factual error: When Sarah's family is fleeing Richmond, the highway sign says "Interstate 66." That highway is about a hundred miles north of Richmond; it starts in Washington DC. Another highway sign says "Virginia Beach 12 miles", which is the wrong way when fleeing Richmond westward.
Plot hole: The government would have no chance of hiding the existence of the comet for a year. Most comets are discovered when they are very faint, literally thousands of times fainter than Wolf-Beiderman when Leo found it. Long before the events in the film take place, probably even before the government itself would be notified, the comet would have been discovered, an orbit calculated, and people panicked. The whole premise of the first half of the movie is thus based on flawed logic.