[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.