1. A vehicle anti-collision device comprising: a rangefinder for mounting inside a following vehicle near to a windshield thereof for measuring a distance of said vehicle from a leading vehicle, distance sampling means coupled to the rangefinder for sampling measured distances at predetermined time intervals, self-speed determination means for measuring a self-speed of the following vehicle, safety time determination means coupled to the distance sampling means and to the self-speed determination means and responsive to the measured self-speed of the following vehicle for determining a safety time between the following and leading vehicles; comparing means coupled to the safety time determination means for comparing said safety time with a predetermined threshold, and alarm means coupled to the comparing means for generating an alarm if the safety time is less than said predetermined threshold, wherein the rangefinder is coupled to a steering mechanism of the following vehicle and includes: a laser light source for emitting a beam of laser light, a first component of which is reflected by the windshield, a second component of which passes through the windshield of the following vehicle so as to be reflected by the leading vehicle, detector means for receiving said first and second components and generating respective first and second detector signals, timing means coupled to the detector means and responsive to the first and second detector signals for measuring an elapsed time between the detection of the first and second components, distance determination means coupled to the timing means and responsive to said measured elapsed time for determining the distance between the following and leading vehicles, turning angle determination means coupled to the steering mechanism for determining a turning radius thereof, lane differentiation means coupled to the turning angle determination means and responsive to said turning radius, to a known dispersion of said laser beam, to a known lane width and to a known vehicle width for determining a maximum distance between the following vehicle and the leading vehicle for which the two vehicles are in identical lanes, and switching means coupled to the lane differentiation means, to the turning angle determination means and to the detector means for intercepting the reflected first component of the laser beam and directing it to the detector means only if the two vehicles are in identical lanes.