This paper introduces a speed adaptation system using roadside-to-vehicle RF communication. The RF transmitter transmits transmitter''s ID, speed limit, message to the drivers. The messages include occurrence of accident, freezing road, existence of gas station, or construction on the road, etc.At present, drivers identify the speed limit from speed sign board. But drivers cannot concentrate his attention continuously on finding speed sign board, so over speed takes place unintentionally. And one of current information medium to the drivers is variable message sign board (VMS) showing the present road status such as severe congestion, occurrence of accident, or heavy rain. But due to high installation cost of VMS, insufficient number of VMS units are installed on the road. So timely notification of traffic accident to the following drivers is impossible now. Our system is low cost to be installed at close intervals on the highway.Each RF transmitter on the roadside in linked to the central command center by wireless system such as beeper or cellular phone for information input. In our system, the driver who encounters any traffic accident reports to the RF transmitter by pushing a button. The RF transmitter then transmits the signal to the central command center where the information is forwarded to the roadside TX one station back, so that following drivers can get information in time. Messages are displayed on an LCD panel as well as through speaker. If the speed of the car exceeds the speed limit of the road, the main controller issues alarm messages and when no action is taken by the driver, the acceleration pedal is automatically controlled until the speed of the car becomes below the speed limit.