How can I calculate my car's speed from a dash cam video?

Question

I have a dash camera on my windshield, and I'm wondering if there is any way to determine my vehicle's speed using the video footage from the dash cam. Please help!

Answer 1

One strategy may be to use the dashed lines in the lane dividers.

A source like this can provide size and spacing. Then mean velocity $v$ can be expressed in terms of, $d$, distance (on the video) and time, $t$, (a watch your arm): $$ \bar{v} = \frac{d}{t} $$

Answer 2

The federal guideline for the white dashed lines on roads is 10 feet, and the empty spaces in between are 30 feet. If you measure the time it takes to move from the beginning of one white dashed line to the next, that is the time it takes your car to move 40 feet, which is a measure of speed.