How to film someone being run over by a car

In the flashback scene I am going to have a woman get run by a car to set up the film.

To film this I thought I could film the car and the person separately and then lay the pieces of film together to create the effect of someone being run over. Unfortunately you can't lay images over each other on the editing software that we are using, iMovie.
Instead, I decided to have a look on YouTube to see if I could find a video which would show me how to film someone being run over by a car. I found it very difficult to find a video which would do this. I did, however, find a video by someone who had previously done this themselves for their own film. The video is actually about a problem that the guy faced when filming this sequence, he broke the windscreen of his car. In his video he has briefly explained how he filmed the shot and the show motion shot is shown within the video so I was able to see how to film someone being run over by a car.
Another idea I had was to film parts of the shot and not have a person in front of the car. I could then put these shots together so that the audience knew what had happened without having to actually see it. I thought this would be a very good way of showing the action as the scene is only meant to be a flashback and the person who is having the flashback doesn't really want to remember the night.

I have decided that I will film the woman being run over by a car in the last way that I described. I have made this decision because I think that trying to film someone rolling over a slow moving car would be very difficult not only to film but to also carry out as an actor. I think that this would be more conventional of a thriller as the audience would be forced to think about what is happening, also the shots, the build up and the character who is witnessing the action would make it quite obvious to the audience what is going on. Being a flashback scene, I don't think the audience need to necessarily be told piece by piece what is going on as, quite often to the person who is having them, flashbacks can be quite disjointed anyway.