This paper describes the development of an adaptive cruise control system (ACC), a driving assistant system, whose purpose is to control the vehicle's speed, determined by a setpoint inputted by the driver. Simultaneously, the system must also monitor the environment and adjust the car's velocity in order to maintain a safety distance from the other vehicles driving on the lane. This system's main purpose is to increase the driver's comfort, thus making it so he will no longer be required to control the throttle pedal and therefore be able to focus on the other tasks involved in driving. For the speed control to be possible, the system communicates with an open engine control unit (ECU), developed by the Automotive Engineering Group (group formed by researchers members of Escola Politécnica da USP and of Faculdade de Tecnologia de Santo André) as well as with an automotive radar, installed on the vehicle's front bumper, responsible for measuring relative distance and relative speed to the other vehicles driving on the road. As the vehicle used does not have brake-bywire technologies implemented, the speed reducing is done via engine breaking. After its development, the system was implemented on a Volkswagen Polo 2004 and its performance was validated with tests on an inertial dynamometer and with road tests.