MUTCD and metric road signs in the US

The MUTCD (Manual on Uniform Traffic Control Devices) from the Federal Highway Administration (FHWA) is adopted by US law as the set of requirements for traffic signs, signals, and road markings in the US.

Although the MUTCD itself uses metric measurements, it doesn't specifically require or forbid metric road signs. However, the MUTCD does specify the format of metric road signs, when used, and some examples are shown below.

Metric road signs in the US

Metric road signs in the US must conform to the MUTCD, and except for speed limits, metric signs generally look just like their non-metric counterparts except for the units, which are written in the usual way:

Metric sign examples

Note that words on signs — except for names of places and streets — are generally required to be all-caps. An exception (although seemingly not explicit) is made for metric unit symbols.

Following are some examples of metric signs from the MUTCD.

Speed limit signs

Speed limits are in multiples of 10 km/h and look like the leftmost sign below. If there's a special speed limit for trucks or the like, an additional sign is included below the first, and similarly if there's a different night speed limit. The signs can be combined in the same panel.

Speed limit signs

History of metric speed limit signs

If you're wondering why the US isn't using the European style speed limit sign, which looks like this,

European speed limit sign

here's the story.

The original US metric speed limit sign, in the 2000 MUTCD, looked exactly like non-metric speed limit signs. As a result, it required supplemental “metric” and “km/h” plaques so you could tell the difference:

Original metric speed limit sign

In 2002, the FHWA proposed replacing that three-part sign with a single sign using a red circle around the speed limit and a “km/h” legend below. (The proposal also mentions a sign using a green circle, but I believe that was a typo.)

However, comments on the proposal expressed concern that a red circle is generally associated with a prohibitory regulatory message, and that a speed limit doesn't fall into that category of message.

So, in 2003 the FHWA decided against the color red but in favor of the concept of placing a circle around a metric speed limit, to provide a clear and easily noticed distinction between metric and non-metric speed limit signs, and changed the color of the circle to black.

Minimum speed limit signs

If there's a minimum speed limit, a sign like the one below is installed below a speed limit sign. Alternatively, both are combined in a sign like the one on the right.

Minimum speed limit signs

Advisory speed limit signs

The advisory speed plaque on the left is used to supplement a warning sign, while the advisory exit, ramp, and curb speed signs may stand alone.

Advisory speed limit signs

Combination horizontal alignment/advisory speed signs

A turn or curve sign can be combined with an advisory speed sign like this:

Turn and advisory speed limit signs

Weight limit signs

Weight limit signs resemble the examples below.

Weight limit signs

On road signs, note that a capital T is the symbol for (non-metric) tons, while a lowercase t is the symbol for metric tons.

Low clearance signs

Low clearance signs show clearances less than 300 mm above the statutory maximum vehicle height, and should show the clearance to the nearest 25 mm (rounded down, of course). The sign on the right is a variant that can be used when mounted on the structure itself.

Low clearance signs

Reference location signs

Reference Location signs — kilometer posts — show integral kilometers if they're installed only at integral kilometer distances:

kilometer posts

Alternatively, “Intermediate Reference Location” signs, which include one decimal place, may be installed at intervals of 0.1 km or a multiple thereof. In this case, the kilometer posts at integral kilometers must include a decimal point and zero.

kilometer posts

Finally, “Enhanced Reference Location” signs include the route number: