(a) Except as provided in subsection (b) of this Code section, the driver of any motor vehicle carrying passengers for hire or of any school bus, whether carrying any school children or empty, or of any vehicle carrying explosive substances, flammable liquids, hazardous waste or constituents, or hazardous acidic liquids as a cargo or part of a cargo, before crossing at grade any track or tracks of a railroad, shall stop such vehicle within 50 feet but not less than 15 feet from the nearest rail of such railroad and while so stopped shall listen and look in both directions along such track for any approaching train and for signals indicating the approach of a train and shall not proceed until he can do so safely. After stopping as required in this Code section and upon proceeding when it is safe to do so, the driver of any such vehicle shall cross only in such gear of the vehicle that there will be no necessity for changing gears while traversing such crossing, and the driver shall not shift gears while crossing the track or tracks.

(b) No stop need be made at any such crossing where a police officer or a traffic-control signal directs traffic to proceed.