Regulatory Signs: Rules You Must Obey
Regulatory signs tell you what you must do, must not do, or must yield to. They are the "law" layer of the sign system, and Arizona questions test whether you can separate a command from a warning. The core family includes STOP and YIELD, speed limit signs, turn and lane-use controls (left turn only, straight or turn), one-way and do-not-enter signs, no-parking and parking-restriction signs, and prohibitions such as no U-turn or no right turn on red.
Most regulatory signs are rectangular with a white background and black legend, though STOP, YIELD, and DO NOT ENTER use their own distinctive designs. Read the sign as a complete instruction: a lane-use arrow tells you what that lane permits, and a turn prohibition applies at that location, not everywhere.
Common traps: confusing a regulatory sign with a warning sign because both are rectangular; assuming a green light overrides a posted "no turn on red"; treating a YIELD as optional when cross traffic is clear; and misreading a speed limit as a suggestion. Another trap is a sign that restricts parking only during certain hours or on certain days - the text below the symbol matters.
Rule of thumb: if ignoring the sign would be a violation rather than merely a hazard, it is regulatory. Worked example: you approach an intersection with a green light and a sign reading "NO TURN ON RED." The light is green, so the restriction does not apply to your turn - but if the light were red, you must stop and wait. The sign modifies the turn-on-red permission, not the green-light permission.