1993 Buick Riviera Owner's Manual - Page 146

A DIAMOND-shaped sign is a warning of something ahead-for example, a curve, steep hill, soft shoulder, or a narrow bridge. . A TRIANGULAR sign also is used..on two-lane roads to indicatea NO PASSING ZONE. This sign will be on the left side of the roadway. '1 A TRIANGLE, pointed downward, indicates YIELD. It assigns the right-of-way to traffic on certain approaches toan intersection. u KEEP RIGHT LEFT OR THROUGH 't [ ONLY j RIGHT TURN ONLY RECTANGULAR (square or oblong) signs show speed limits, parking regulations, give directions, and such information as distances to cities.