rule of the road

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A set of customary practices, especially for the operation of a motor vehicle, boat, or aircraft, established to promote efficiency and safety. Often used in the plural.
2. n. A set of established rules or practices, usually unstated, intended to minimize confusion or conflict. Often used in the plural: "Great powers ... wishing to avoid war could resort to one of three tools: condominium, spheres of influence or rules of the road” ( Charles William Maynes).

Wiktionary

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

GNU Webster's 1913

1. n. (Law) any of the various regulations imposed upon travelers by land or water for their mutual convenience or safety. In the United States it is a rule of the road that land travelers passing in opposite directions shall turn out each to his own right, and generally that overtaking persons or vehicles shall turn out to the left; in England the rule for vehicles (but not for pedestrians) is the opposite of this.