Fun. Johnny555 is right of course, and it was the Jeep hacked on the Interstate last year that first inspired me. The airplane scenario may be more likely, but I liked the additive nature of so many cars, cars a bit like insects. (You could go further in this direction by conjuring up multiple drone swarms.)

As to movie points: of course Will Smith is the hero, and we'll handle DC rush hour stasis through special effects. ;-)