The open auto transport alternative is the most common one.  If you have spent any time on US highways you have already seen such trailers at work. These trailers have one or two open air levels on which cars are loaded and secured.

Open auto transport advantages:

This option is cheaper than the enclosed transport alternative.
Such trailers are more readily available.
Disadvantages:

Your car is exposed to the hazards of the road (rocks, dust, branches) and weather.  However, in the vast majority of cases, damage caused by these elements is unlikely.