This paper describes the development of a radar trap warning system that can be used in road traffic. A server, realised in Java, provides position data of radar traps over the gsm network to the mobile warning device, mounted in a vehicle. Additionally, the driver has the possibility to report a mobile radar trap by pushing a button. These radar traps are stored in the server's database and are sent to all other warning devices. The warning device hardware is based on the Falcom's XF55-AVL module, that consists of a GPS-receiver, a GPRS modem, an ARM7TDMI processor and 2 megabytes of flash memory, all located on a single hardware with very small physical dimensions. The software of the warning device is written in C and C++ and is based on the opensource embedded operating system eCos. To determine if there is a radar trap near the driving vehicle, the system uses non-trivial mathematical algorithms. In this paper, these algorithms are optimized in order to be simple and efficient. A data structure for storing radar traps in the flash memory has been defined in order to achieve this goal. It can be efficiently accessed using c-structs. For testing and demonstration purposes, a GPS simulation software using SwissMap50-files as input has been created. Also a part of this paper is the study of Swiss local laws to clarify if such a system is legal or not.