Why don't american speed limit signs have a unit?

American speed limit signs look very different from the rest of the world. Unlike Europe and most countries that display both the number and the unit (km/h), U.S. speed limit signs simply show a number — without “mph.” This often confuses travelers, but there is a simple reason behind it: the United States uses only one speed measurement on its roads, and the official road sign standards don’t allow units to be displayed.

Why U.S. Speed Limit Signs Don’t Show Units (Complete Explanation)

If you’ve ever seen American speed limit signs, you’ll notice they look extremely simple:

SPEED LIMIT
45

• No “mph”
• No “km/h”
• Just a number

For many visitors, this feels incomplete — but there’s a clear logic behind it.

1. The U.S. Uses Only One Road Speed Unit

In the United States, all road speeds are measured in miles per hour (mph). There is no second system used anywhere on U.S. highways, so units are considered unnecessary.

2. MUTCD Rules Do Not Allow Units on the Sign

U.S. road signs follow the MUTCD (Manual on Uniform Traffic Control Devices), the official rulebook for all traffic signage.

The MUTCD requires:

• The sign must include the text “SPEED LIMIT”
• The speed value must be shown as a number only
• Units like “mph” must not be added

This keeps signs simple and consistent across all 50 states.

3. Europe Uses km/h — So Units Are Required

In most European and metric-system countries, speed limits look like:

• 50 km/h
• 80 km/h
• 120 km/h

Because many European countries share borders, drivers constantly move between regions. Showing the unit avoids confusion between mph and km/h.

4. Visual Design Differences Between U.S. & Europe

U.S. Speed Limit Sign

• Rectangular shape
• Black text on white background
• Shows “SPEED LIMIT” + number
• No unit displayed

European Speed Limit Sign

• Circular shape
• Red border
• Black number
• Often shows km/h

5. Will the U.S. Ever Add Units?

Very unlikely. The U.S. has used mph for centuries, and switching to km/h would cost billions and require massive infrastructure changes.

6. Why This Confuses Tourists

Travelers often ask:

• “Is this 45 mph or 45 km/h?”
• “Why don’t American signs show units?”
• “Why do U.S. speed limit signs look so plain?”

This is why blogs like this help drivers understand the differences.

Conclusion

American speed limit signs don’t show units because:

• The U.S. exclusively uses mph
• MUTCD rules require a simple “SPEED LIMIT” + number display
• Drivers never need to choose between mph and km/h

In contrast, Europe and most metric countries display km/h to avoid confusion across borders.

FAQ (Frequently Asked Questions)

1. Why do American speed limit signs not have “mph”?

Because the U.S. only uses miles per hour. There is no alternate speed system, so units are unnecessary.

2. Is the U.S. the only country that doesn’t show units?

Mostly yes — a few mph countries also omit units, but the U.S. is the most well-known example.

3. Are Americans confused by unit-less signs?

No. Locals grow up learning that all speed limits are in mph.

4. Are tourists confused?

Yes, especially visitors from Europe who expect to see km/h on signs.

5. Is it legal to put “mph” on a U.S. sign?

No. MUTCD rules prohibit adding units.

6. Will the U.S. ever switch to km/h?

Very unlikely due to high cost and longstanding infrastructure.

7. Why do European signs have a red circle?

Because their traffic standards classify speed limits as regulatory signs, which traditionally use red-bordered circular designs.