(a) Except as provided in subsection (b) of this Code section, the driver of any motor vehicle carrying passengers for hire, any bus, whether or not operated for hire, or of any school bus, whether carrying any school children or empty, or of any vehicle carrying any hazardous material listed in Section 392.10 of Title 49 of the Code of Federal Regulations as those regulations currently exist or as they may in the future be amended or in regulations adopted by the commissioner of public safety, 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 or she 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:

(1) Traffic is directed to proceed by a police officer, a firefighter, or a railroad flagman;

(2) A traffic-control signal directs traffic to proceed;

(3) The highway crosses an abandoned railroad track which is marked with a sign indicating its abandoned status, where such signs are erected by or under the direction of the local or state authority having jurisdiction over the highway; or

(4) The highway crosses an industrial siding or spur track marked "exempt," where such signs are erected by or under the direction of the local or state authority having jurisdiction over the highway.