Ok, I have a problem and need a simple explanation of how to calculate something.

I would like to know how to calculate the speed of a vehicle based on two frames from a video recording. You can measure the width of the vehicle in both frames, you can measure the car itself and you know the precise time between each frame, so I presume there is some way to tell how fast it was going? Assume no information is available about the properties of the lens. The distance to the object in the first frame is available, rounded to the nearest metre.

I have done a Google search but there isn't anything that somebody with basic mathematical skills could follow.

edit: Here is an example of the imagery available:
http://www.freepichosting.com/Albums/421617291/1.html
The "NR" is the field number, and it is PAL so there are 50 fields per second.

Gareth