A control device traffic is a system used to regulate the flow of traffic. RADAR (5 letters) is a detection system that uses radio waves to determine the range, angle, or velocity of objects. Servo (5 letters) is an automatic device that uses error-sensing feedback to correct the performance of a mechanism. REIN (4 letters) is a strap or rope attached to the bridle or bit of a horse, used to guide or restrain the animal. All three of these devices are used in control device traffic systems to help regulate the flow of traffic.