Trivia: If you happen to have the DVD version of the film, have a look at one of the trailers. In the movie trailer they show the comet hitting the ocean at an almost right-angle. In the actual movie the comet enters the atmosphere at an angle, thus letting the people watch the comet travel to across the sky into the ocean.
Trivia: When everyone gets out of their cars in the traffic jam and watches the comet pass overhead, they were really looking at a crew helicopter passing over. In post production, the helicopter was covered up with the comet.
Trivia: A sorta nice inside joke: At one point when the President is speaking we see a crowd watching the broadcast in New York City's Times Square. A long shot from above shows the crowd in the streets, and a nearby cinema's marquee shows that the cinema is playing "Fire In the Sky". "A Fire In the Sky" is the title of a movie from 1978 about a comet that hits Phoenix. But this is where it gets sorta tricky: I looked the title up, and noticed something: the title of the movie, notice, is "A Fire In the Sky", while the marquee read simply "Fire In the Sky". That is the name of a movie about the supposedly true story of a guy that was abducted by aliens. A inside joke that doesn't work, due to bad research (or lettering).
Trivia: In a recent American TV airing of the film (January 2004), any scene depicting the former World Trade Center towers were removed, due to existing September 11 sensibilities.
Answer: There is no logical reason. It can be argued that people are just stubborn and/or reluctant to leave their homes under any circumstance. However, this is a movie, and showing the last-minute chaos of people attempting to evacuate heightens the tension and the drama.
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