1. A collision avoidance system for a vehicle comprising: at least two laser radar sets being mounted on at least one of the vehicle's four sides, including a front side, a rear side, a left side and a right side; wherein on each equipped side, at least one laser radar set is mounted near one end of the equipped side and at least another laser radar set is mounted near the other end of the equipped side of the vehicle, each laser radar set including at least one laser radar unit; wherein each laser radar unit includes a transmitter means for transmitting laser radar signals and a receiver means for receiving laser radar signals reflected by a detected obstacle from the transmitted laser radar signals, and generating first electronic signals representing the reflected laser radar signals thereof; a computer means for receiving and processing the first electronic signals from each laser radar unit to determine the distance of the detected obstacle and the obstacle's radial component of relative speed along the laser beam direction, said relative speed of the obstacle being relative to the vehicle; wherein the computer means generates second electronic signals representing the distance and the radial component of the relative speed of the obstacle; an angle message means functionally associated with each laser radar unit for providing third electronic signals representing the emitting or receiving direction of each associated laser radar unit; a steering wheel rotation sensor means for detecting rotation information of a steering wheel of the vehicle and generating fourth electronic signals representing steering wheel rotation information; a speed sensor means functionally connected with a speedometer of the vehicle for generating fifth electronic signals representing the vehicle's speed relative to the earth; the computer means further receiving and processing information from the first, second, third, fourth and fifth electronic signals, wherein the computer means processes the received information to determine when a collision is imminent; an alarm means functionally connected to the computer means for producing an alarm upon receipt of a signal from the computer means indicating a determination by the computer means that a collision is imminent; an extrinsic condition input means for generating sixth electronic signals representing an extrinsic condition; wherein the computer means receives and processes input information from the first, second, third, fourth, fifth and sixth electronic signals and the computer means processes the received information to determine when a collision is imminent; wherein the computer means includes means for processing the second and the third electronic signals from at least two of the laser radar sets to determine the direction of movement and the speed of the obstacle relative to the vehicle and thereby to generate seventh electronic signals representing the direction of movement of the obstacle, and eighth electronic signals representing the relative speed of the obstacle; wherein, by counting the number of laser radar beams being reflected back by the obstacle, the computer means calculates the estimated size of the obstacle and generates ninth electronic signals representing the size of the obstacle; wherein the computer means further includes means for processing the second, third, fourth, fifth, seventh and eighth electronic signals to determine speed and direction of movement of the obstacle relative to the earth and to generate tenth electronic signals representing the speed of the obstacle relative to the earth, and eleventh electronic signals representing the direction of movement of the obstacle relative to the earth.