Section 15. Except as hereinafter otherwise provided, every person operating a motor vehicle, upon approaching a railroad crossing at grade, shall reduce the speed of the vehicle to a reasonable and proper rate before proceeding over the crossing, and shall proceed over the crossing at a rate of speed and with such care as is reasonable and proper under the circumstances. Every person operating a school bus, or any motor vehicle carrying explosive substances or flammable liquids as a cargo, or part of a cargo, upon approaching a railroad crossing at grade, shall bring his vehicle to a full stop not less than fifteen feet and not more than fifty feet from the nearest track of said railroad, and shall not proceed to cross until it is safe to do so. The operator of a school bus, in addition to bringing his vehicle to a full stop, as aforesaid, shall open the service door, ascertain if he may cross safely and thereupon close said door before proceeding. Every person operating any motor vehicle, upon approaching at grade a railroad crossing protected by red lights which flash as a warning, shall bring his vehicle to a full stop not less than fifteen feet and not more than fifty feet from the nearest track of said railroad and shall not proceed to cross until said lights stop flashing. Every person operating any motor vehicle, upon approaching at grade a railroad crossing protected by a lowered automatic gate, shall bring his vehicle to a full stop not less than fifteen feet and not more than fifty feet from the nearest track of said railroad and shall not proceed to cross until said automatic gate is raised. Every person operating any motor vehicle, upon approaching at grade a railroad crossing protected by a railroad employee waving a red flag or white lantern, shall bring his vehicle to a full stop not less than fifteen feet and not more than fifty feet from the nearest track of said railroad and shall not proceed to cross until said railroad employee signals that it is safe to do so. A railroad train approaching within approximately one thousand five hundred feet of a highway crossing shall emit a warning signal audible from such distance. Whoever violates any provisions of this section and is operating a school bus, or any motor vehicle carrying explosive substances or flammable liquids as a cargo or part of a cargo, shall be punished by a fine of not less than $500 or by being required to perform a total of 100 hours of community service which may include service in the operation lifesaver program. All other persons violating the provisions of this section not operating a school bus, or any motor vehicle carrying explosive substances or flammable liquids as a cargo or part of a cargo, shall be punished by a fine of not less than $100 nor more than $200 or by being required to perform a total of 50 hours of community service which may include service in the operation lifesaver program.