What does a yellow advisory speed sign on a curve mean?

Yellow advisory speeds (typically square or diamond yellow signs with a speed number) are recommendations for navigating an upcoming curve, ramp, or hazard. They are not legal limits in the same way black-on-white speed limit signs are — but driving above the advisory speed and crashing is strong evidence of unsafe driving. For passenger cars in good conditions, the advisory is calibrated to be a safe maximum; for commercial vehicles, especially tankers and high-profile vehicles, the safe speed is lower. In wet, icy, or otherwise compromised conditions, the safe speed is lower for any vehicle. The basic speed law requires you to drive at a speed safe for conditions, which may be below the advisory.