This is kind of a tricky question, because the yellow road signs showing a certain speed in black letters are not actually speed limit signs. Technically, those are warning signs. i.e. according to Manual on Similar Traffic Control Devices (MUTCD) – Chapter 1C.02.289 – They are “to inform road users of a situation which may not be readily apparent.” More specifically, black-on-yellow speed signs are commonly used in two applications: as advisory exit and entrance ramp speed signs (2C.12) or advisory speed placards (2C.59). As for their black-on-yellow color theme, it is mandated in Chapter 1D.05 along with the color meaning of other traffic signs in the country, such as green highway signs.
We’ll see below what differentiates those advisory signs from each other, but first, let’s tackle one key difference between those two and the more common black-and-white regulatory speed limit signs: As their name suggests, warning signs are meant only to advise drivers, not to mandate legal speeds. Regulatory signs indicate legally regulated speeds set by traffic engineers. Therefore, you may not get a particularly fast quote for more than the posted speed of an advisory sign.
That said, remember that violating “basic speed laws” can still get you a speeding ticket in almost any scenario. Section 257.627 of Michigan Code of Laws It mandates that vehicles should be operated “at a careful and prudent speed, neither more nor less than what is reasonable and appropriate, having regard to the traffic, surface and width of the highway and any other conditions prevailing at the time.” Needless to say, this may also give the police an excuse to stop minority drivers.
When are yellow speed limit signs used?
As mentioned, the first case for yellow advisory speed signs is climbing and descending highway ramps, as covered in MUTCD 2C.12. These can be standalone signals, or they can be used in combination with others, and there are strict guidelines on when to deploy them. This depends on the speed difference between nearby main roads and the appropriate ramp speed. For example, if state traffic engineers recommend ramp speeds 20 mph higher or lower than the regulatory speed limits of nearby main roads, a black-on-yellow advisory speed sign should be used. With differential speeds of 15 mph and 10 mph, the signals are considered recommended and optional, respectively.
The second situation where you might see a speed sign with a yellow background is when it is used in conjunction with a warning sign for a specific hazard – if you’re suddenly approaching an unexpected sharp turn, you might see a black-on-yellow sign with a turn graphic, as well as a second sign with the advised speed for the turn. The MUTCD refers to these as advisory speed strips in Chapter 2C.59, and ignoring them can be a recipe for an accident.
Naturally, the MUTCD also provides criteria for when these signs are required. Taking our example of a sharp curve, you compare the road’s “regular” speed limit – whether by statute or the 85th percentile rule – to the advisory speed set by the engineer specifically for the turn. Here, speed signs are required when the difference is above 15 mph, recommended above 10 mph, and optional above 5 mph.
