Except in cities or towns, the driver of any motor vehicle carrying passengers for hire, or of any vehicle carrying explosive substances or flammable liquids as a cargo or part of a cargo, before crossing at grade any railroad track, shall stop such vehicle within fifty feet but not less than fifteen feet from the nearest rail of such railroad and while stopped shall listen and look in both directions along the track for any approaching train, and for signals indicating the approach of a train, except as hereinafter provided in this section, and shall not proceed until he can do so safely. After stopping and upon proceeding when it is safe to do so, the driver of any vehicle shall cross only in such gear of the vehicle that there will be no necessity for changing gears while traversing the crossing.

Before crossing any railroad tracks at grade, the driver of any school bus shall stop the school bus within fifty feet but not less than fifteen feet from the nearest rail of such railroad and while stopped shall listen and look in both directions along the track for any approaching train, except as hereinafter provided in this section, and shall not proceed until he can do so safely. After stopping and upon proceeding when it is safe to do so, the driver of any school bus shall cross only in such gear of the vehicle that there will be no necessity for changing gears while traversing the crossing.

Notwithstanding the foregoing provisions of this section, no stop need be made at any such crossing where a law-enforcement officer or a traffic-control signal directs traffic to proceed.