Definitely. The code here assumes that the vehicle's vector is perpendicular to the optical axis. If you had enough information to calculate the vector it would just be a little extra trig to measure the relative speed minus the offset of your vehicle's instantaneous speed. This is easy for side to side movement, a little harder in the depth direction - though you could get some depth info by comparing the size of the car contour at two or more snapshots (smaller indicating that it's moving away). The framerate/resolution of the RasPi setup probably wouldn't make it a good choice for this though.

So you definitely need to know your own speed to calculate relative speed of other drivers?

I have a dumb dash cam that's just awesome for video but is nothing more than video to a sd card, ie, not a computer. I've always wondered if I could use just the video using familiar objects like stop signs, mile markers or lane markers to determine my own speed.

If you had an OBDII adapter and a camera this should be possible! I'm not sure if there is OBDII communication code for RPI, though

There is. There are obd boards that use the standard uarts, and dongles that use bluetooth that you can read from the pi. Here's one guy's project on it. http://fabcirablog.weebly.com/blog/reading-a-cars-obdii-port...