rule of the road

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A set of customary practices, especially for the operation of a motor vehicle, boat, or aircraft, established to promote efficiency and safety.
• noun A set of established rules or practices, usually unstated, intended to minimize confusion or conflict.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The regulation requiring all traffic (travelling on a road or otherwise) to keep either to the left or the right.
• noun nautical A regulation concerning safe handling of vessels on seaways.