In North America, the u.n.'s transportation of dangerous goods-model regulations is the basis of harmonized regulations for all modes of transport.
Transportation within the United States is facilitated by road, air, rail, and water. Most passenger travel is by car for short distances and by air (or rail in some regions) for longer distances. In descending order, most cargo moves by rail, truck, pipeline, or boat. Airmail is generally only used for perishables and express shipments. Transportation is the largest source of greenhouse gas emissions in the United States.
Transportation safety is regulated by several transportation authorities and other federal agencies. Road safety in general, and vehicle safety in particular, is the responsibility of DOT's National Highway Traffic Safety Administration (NHTSA).
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