Not merely running at the designated constant speed as the classical cruise control, the adaptive cruise control maintains safe headway distance when the front is blocked by other vehicles. One of the most essential part of adaptive cruise control system is the range sensor which can measure the position and speed of all objects in front continuously, ignore all irrelevant objects, distinguish vehicles in different lanes and lock on to the closest vehicle in the same lane. In this paper, the rule-based object recognition algorithm is proposed to process raw laser scanning data and obtain valid distance to target vehicle. The algorithm proposed has two key concepts : life index and level transition. The concept of life index qualifies the reliability of the recognized object and the concept of level transition gives us robust and reliable data under critical cases when other noisy objects interfere a target object.