A system for monitoring operation and location of a first moving vehicle relative to a second moving vehicle. A minimum separation distance between the first and second vehicles is estimated, based on the first vehicle velocity, and optionally on the second vehicle velocity, using location determination (LD) signals received from satellite-based transmitters from GPS, GLONASS and LEO satellites, or from ground-based signal sources such as LORAN signal towers, and using ranging signals from SONAR, RADAR or a similar system. The minimum separation distance is compared with the actual separation distance at selected times, and a vehicle driver is advised if the actual separation distance is too small, if the separation distance is decreasing too quickly, or if the second vehicle velocity is decreasing too quickly. The second vehicle may travel in the same traffic lane, in an adjacent lane, or on a road that intersects the road used by the first vehicle. Where the first and second vehicles travel on separate roads that will intersect, the system estimates whether the second vehicle will stop, or will be able to stop, at the intersection. The second vehicle may be a railroad car, such as a locomotive, or a road vehicle. A maximum vehicle clear-view velocity, consistent with vehicle stopping within a selected distance, is estimated. Road conditions are estimated and compensated for in estimating the minimum separation distance and/or the maximum vehicle clear-view velocity.