You may have noticed different types of speed limit signs while driving without thinking much about it. The common black-and-white rectangular sign posts the maximum speed required by law. A yellow (or sometimes orange) speed sign is different: it’s advisory, not mandatory.
A regulatory sign states a law drivers must follow. An advisory sign recommends a safer speed for a specific stretch of road. The two are easy to tell apart: regulatory signs are black and white, while advisory signs are yellow or orange with black lettering. Advisory signs almost always have a yellow background, and they’re sometimes rectangular instead of square so a word like “ramp” or “curve” can sit above the speed value.
Advisory signs show up most often ahead of sharp curves, off-ramps, and other spots where conditions change quickly, plus anywhere severe weather, heavy traffic, or another hazard calls for a lower speed than the posted limit.
The Federal Highway Administration sets the broad rules for traffic signs, but states and local governments set the actual speed limits for their roads. States generally set limits on freeways; cities set them on local streets, within the bounds of state law.
Advisory speeds specifically are based on physics, not statute. Traffic engineers use a tool called a ball bank indicator to measure the force a vehicle experiences moving through a curve, on a scale of zero to twenty degrees. A reading around ten degrees means drivers can adjust speed and stay in control without much trouble; twenty degrees signals a much stronger force that calls for more caution. The MUTCD requires an engineering study before an advisory speed sign goes up, so the number on the sign reflects that measurement, not a guess.
In most cases, driving faster than an advisory speed carries no direct penalty. It’s framed as a recommendation, not a law. But that doesn’t make it optional in every situation: if you’re involved in an accident while exceeding an advisory speed in the area it covers, you can still be cited or found liable for violating the basic speed law, which requires driving at a speed reasonable for the conditions regardless of the posted limit.
Every state sets its own default speed for situations with no sign at all. In California, for example, the default limit at a blind intersection with no stop signs and less than 100 feet of visibility is 15 mph, and the same 15 mph default applies in alleys. States also generally require drivers to slow down for heavy traffic, poor weather, or children nearby, sign or no sign. Most drivers don’t know these defaults, which is exactly why posted regulatory and advisory signs matter: they remove the guesswork.
Not every advisory speed is permanent. For temporary conditions, agencies increasingly use portable, battery-powered electronic signs instead of installing a fixed sign. Zumar’s TraffiCalm line covers this: dolly-mounted, LED speed feedback signs that can be rolled into place for a work zone, an oil spill, a mudslide, or a stretch of fog or heavy rain, then moved out again once conditions clear. They can run static text or flashing alerts, and some display a driver’s own speed in real time using radar, which tends to get compliance that a static sign can’t.
An advisory speed limit is a recommendation for driving slower than the posted or default limit; a regulatory sign is the law. Both exist to cut down on guesswork and keep drivers safe through curves, ramps, and changing conditions. Contact Zumar at our Arizona, California, or Washington location for advisory signage, TraffiCalm electronic speed signs, or help figuring out which fits your site.