The exterior of the car crash was done as a composite shot. First shot was seeing the main car driving toward the intersection, then panning over to see the other car. Then we did a separate shot of the main car whizzing through the intersection. I composited the main car running through the intersection on top of the shot of the other car.
Then we cut to the interior, which I basically did as a frame-by-frame thing. I shot at a very low shutter speed and moved the camera around while exposing each frame, to get that blurred motion look (as it would look if something was happening very fast). I had them slowly move backward into their seats, which looks more like they're flying out of their seat when the footage is played backward. Then I did a frame by frame of the airbag popping. I added blood and shattered glass digitally.
I love coming up with stuff like that. I wrote it because I knew I could make it work, and that it would be effective. And it didn't cost anything!
As far as production, we'd shoot for small periods of time over the course of about 1.5 months, more or less only shooting one scene or two scenes at a time, or sometimes only part of a scene. The movie bounces around a lot of locations, and it worked out better for us to shoot one location at a time.