A roll on roll off ship, or ro-ro, is a ship which has the ability to take on and release vehicles without making any special arrangements for the vehicles, hence the term "roll on, roll off."
The first roll on roll offs were established in the mid 19th century with the advent of the railroads system. Ships would be equipped with rails that could be connected to onshore rails. A train could then be loaded directly onto the ship and carried across bodies of water too wide for bridges.
It took nearly a hundred years for this technology to evolve and allow general road-ready vehicles to board ships. During World War II, the Army took notice of a Japanese innovation on their landing craft: a small bow ramp allowed troops to quickly abandon the boat and deploy to land. They began developing their own craft in this manner, and finally Andrew Higgins, a New Orleans boat builder, developed an amphibious landing craft that could come completely ashore without damaging itself. These Higgins boats were then improved by adding the bow ramp, and became very popular for deployment in the Pacific and European theaters.
Eventually, this technology was expanded to include landing craft for tanks and other combat vehicles. The Mk4, LCM, and LCI all served admirably in their task, and the roll-on, roll-off technology that supported them also began to see its first non-military use at industrial ports across the world. By the 1950s, Higgins and other boat manufacturers were developing larger ferries to transport large amounts of cars across wide watery expanses.
Generally, ro-ro ferries come in two flavors: the local domestic ferry, for carrying tourists and passengers across small bodies of water to continue their journey; and large commercial ferries, for transporting large amounts of cars across the oceans for private owners and dealerships. In 1994, over 4,500 of these such ferries were in operation, and carried nearly 20 million cars every year.
- www.oceansatlas.org/unatlas/uses/transportation_telecomm/ maritime_trans/shipworld/cargo_car/roro/ro_ro_ships.htm