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

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.