If you have a speed problem, here’s a  (difficoulty level:9) tutorial by nootropic design, about making a custom speeding alert for your car.

OK, it’s me, but you already figured that out. If you weren’t smart, you wouldn’t be here. Here’s a rundown of this project’s features:

• A simple perf-board Arduino circuit with an EM-406a GPS Receiver connected to the microcontroller
• A pair of CAT-5 cables running from the circuit to the back of my Honda Civic
• A small perf-board with red and blue LEDs and two white LEDs representing the headlights of a police car. This small board is mounted on the inside of the rear windshield using suction cups.
• The software running on the microcontroller is programmed to know the speed limit in different locations near where I live and drive. I did this by specifying “speed zones” which are polygons and a speed limit. The polygons are defined as a list of latitude/longitude vertices.
• Whenever the GPS module reports the current position and speed (every second) the code determines which bounding polygon or “speed zone” the car is located in. If the GPS receiver reports that the current speed is greater than the speed zone’s limit, the police lights are activated. If below the speed limit, the lights will be turned off.

Via [nootropicDesign]