[votingprawn]

I disagree with you regarding resolution and frame rates, I don't see why you would need a high speed camera. Something like a raspberry pi camera would be perfectly sufficient for this, assuming you still just want to get someone's speed to within say +/- 1mph to shame them, as in the original comment.

Set up your RPi camera about 20m from the road, with it's FOV that'll give you about 20 m of road coverage. Rule of thumb in my world is that the items of interest should not move more than 1/4 of window width between frames. So that's 5m, to move 5m between frames at 30Hz you'd need to be doing about 300 mph. More realistically the car might be doing 25m/s giving you an inter-frame displacement of 0.8m ~130px. If that's too little for your software to register just up the inter-frame time.

[logfromblammo]

You only need high-speed if you're mounting a device immediately adjacent to the road with a low-resolution light sensor.

An ordinary surveillance camera certainly can estimate speed, especially with good software, but a dedicated device would likely give a better performance to price ratio or performance to power ratio. Maybe I'm just too old, and still think that a RPi and enough resolution for a human to actually discern an image is overkill, because I'm used to computing power and image resolution being expensive.

But then again, if you're already using that setup to read license plate numbers, you're not actually starting from zero. The marginal effort to estimate speed on top of that is likely less than a purpose-built device.