The World History Encyclopedia has a great infographic by Simeon Netchev demonstrating how Roman roads were built, layer by layer.

“An infographic illustrating the typical structure of Roman roads. It is believed that the Romans adopted the craft of road construction from the Etruscans, and as the empire grew, they incorporated ideas and techniques from other cultures.

At its height, the Roman transportation network spanned 85,000 km (53,000 miles) of paved roads extending from Mesopotamia to Britain and from the Danube to Spain and North Africa. The engineers of the Roman army followed a uniform set of fundamental principles while adapting to local materials and conditions.”

The solid rocket boosters (SRBs) on the sides of NASA’s current SLS Artemis rockets and former space shuttles are said to be the same width as a Roman chariot traveling on Roman roads. The SRBs are 8 feet 4 1/2 inches wide, or about 1.4 meters. So why the odd size? They are transported by rail from manufacturing to the Kennedy Space Center in Florida and that’s the largest width cargo. And we can read that:

The US Standard railroad gauge (distance between the rails) is 4 feet, 8 1/2 inches.

That’s an exceedingly odd number. Why was that gauge used?

Because that’s the way they built them in England, and the US railroads were built by English expatriates.

I see, but why did the English build them like that?

Because the first railway lines were built by the same people who built the pre-railroad tramways, and that’s the gauge they used.

Well, why did they use that gauge in England?

Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing.

Okay! Why did their wagons use that odd wheel spacing?

Because, if they tried to use any other spacing the wagon wheels would break on some of the old, long distance roads. Because that’s the spacing of the old wheel ruts.

So who built these old rutted roads?

The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The Roman roads have been used ever since.

And the ruts?

The original ruts, which everyone else had to match for fear of destroying their wagons, were first made by the wheels of Roman war chariots. Since the chariots were made for or by Imperial Rome they were all alike in the matter of wheel spacing.

Thus, we have the answer to the original question. The United States standard railroad gauge of 4 feet, 8 1/2 inches derives from the original specification for an Imperial Roman army war chariot on Roman roads!

Check out these other related posts:

• If the ancient Romans had Google Maps
• The Roads of the Roman Empire as a Subway Map