Lego Mindstorms. Input used the ultrasound sensor. Used some basic math (distance over time) for the calculation. And the LCD display gave the output.
There are lots of variations that could be added to the project. I found that meters per second was easier to compute than miles per hour. We have an older educational kit that includes an LED and transparent bricks, so it could also light up if a speed limit was exceeded.
Download the build instructions and code here.