All vehicles carrying passengers for hire, all buses carrying passengers and all vehicles carrying explosives or flammable liquids must always stop no closer than and no farther than from railroad tracks.

15 feet, 50 feet

Explanation:

The distance requirement is made due to the huge risk that this type of vehicles carry. (Death or large damages to the nearby area). The distance requirement could vary depending on the states where you live in, but this distance is applicable in majority of the states in USA.

Generally, vehicles that carried a lot of passengers or explosive substances tend to move on a slower pace compared to normal vehicles. Stopping Distance between 15 - 50 feet before crossing the railroad is considered to be the safest distance that has the lowest probability of collision.

i would say b. research typically uses a variety of sources

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