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The road is to be used to resupply a city siege.

The best estimate of speed for how fast the Roman's could build a road I can find is their construction of the Appian Way or "Queen of Roads". This road is 20 feet wide and 132 miles long, and was completed in approximately 11 years, translating to a construction speed of one mile a month.

I don't have information on the size of the work crew or if they worked at night at all, but I'm going to assume work time on the road being equivalent to our 8 hour work day, the rest of the time being spent training, eating, etc...

I assumed a proportional number of workers would be building roads on my world. The population of the Roman Empire was estimated at 56.8 million, and the population on my world focussing on building the road is 500 million. Which equates to 8.8 times faster for each construction lead. If it was only one road construction lead, 2 years 10 months to finish.

War time speed I assume to be 24/7 construction by three construction teams. That reduces the time to 1/3 at 11 months and 13 days. My world has resource locations enough for 3 construction leads, which brings the final completion speed down to just 3 months and 24 1/2 days. Does that speed sound reasonable?

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    $\begingroup$ I doubt the Romans worked at night. Probably road gangs worked sunup to sundown. $\endgroup$
    – elemtilas
    Commented Jun 10, 2020 at 2:29
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    $\begingroup$ Via Appia was built in stages, in the late 4th to mid 3rd century before the common era. The extent of the Roman power at that time was nowhere near what it would become in the 1st century. The population of the Roman republic in the 3rd century BCE was very much smaller than 57 million. (And roads are expensive. Nobody in his right mind even considered building a hundred meter wide road in the days of the Roman empire. It would have had no practical purpose whatsoever. What is your intended use that super-wide road in a world with Roman technology?) $\endgroup$
    – AlexP
    Commented Jun 10, 2020 at 2:34
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    $\begingroup$ Why one single, gigantic and oversized road? Why not a network of normal roads? Your besieged city is at least 20 km in diameter, the far side of the siege would get their supplies way too late, it would spoil quite quickly. Also, it is quite unlikely that all supplies come from one place. So, sorry, but the whole concept is rather flawed. $\endgroup$
    – Erik
    Commented Jun 10, 2020 at 6:19
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    $\begingroup$ As a frame challenge: a world in which nine million soldiers are laying siege to a single city is not going to resemble the medieval world economically, politically, technologically, or militarily. For comparison, Genghis Khan's horde was probably about 100,000 strong at its greatest extent, and that was spread over the empire. There were about 7.5 million Allied troops involved in the entire European theater of WW2. $\endgroup$
    – Cadence
    Commented Jun 10, 2020 at 9:38
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    $\begingroup$ For reference, a 300-foot wide road is the equivalent of a 25-lane highway, and would be considered a very, very big road even in modern times. This seems excessively large for such a purpose. $\endgroup$
    – Nuclear Hoagie
    Commented Jun 10, 2020 at 15:39

2 Answers 2

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This Answer (from History SE) might be of some use.

The long and short of it is between 1 1/2 to 2 yards of 16 foot wide road per man per day.

According to the Twelve Tables, roads were to be constructed 8 foot wide (where straight) and 16 foot wide (where curved). Later it became common for roads to be 12 foot wide, which would accommodate the passing of two carts with sufficient room for foot traffic to not be disturbed --- essentially a "two lane road".

You want a road twenty-five times as wide as a Roman road (or eighteen times the width of the Scottish built Roman-style road): I guess a sort of Roman superautobahn. A three hundred foot wide road would accommodate twenty-five lanes of lanes of traffic: Roman waggons were built at four foot gauge (approximately the same as modern "standard" gauge railways).

The rest is just a matter of mathematics: if it takes one man one day to make a segment of road 2 yards long by 5 yards wide, it would take the same man 20 days to make the same distance but 100 yards wide (5yd x 20 days = a road segment 100yds wide by 2 yards long). Or alternatively, a gang of 20 men can do that in one day.

If you have a 500 man crew (25 gangs of 20 men), plus all the additional civilian labour as described in the other answer, you can make your 100 yard wide road go 50 yards forward in a day. 300 miles is 528,000 yards. Your crew will finish the job in 10,560 days - about 29 years.

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  • $\begingroup$ The problem with the man * yards / day is that it only works until people start running into each other. Eventually, you get to a point where adding extra men causes the project to take longer. $\endgroup$
    – NomadMaker
    Commented Nov 15, 2020 at 6:46
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    $\begingroup$ @NomadMaker You don't have to put all the people into a single road crew. Depending on your engineering ability, you could have crews spaced at intervals of a hundred yards to many miles. Perfect linear scaling isn't realistic, but you can mitigate some of the worst factors with planning. The further you can space your crews, the less interference they'll cause for each other, but you also have to consider logistics (feeding one large camp allows some economy of scale vs feeding many smaller ones) $\endgroup$
    – Extrarius
    Commented Nov 15, 2020 at 15:22
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Instead of a Roman road, build a plank road.

https://en.wikipedia.org/wiki/Plank_Road_Boom

The Romans built roads to last indefinitely. You don't need that. If you have ample wood you can cut it down and build a plank road, fast. I knew there were plank roads in the US but I had no idea how extensive they were until I read the linked Wikipedia.

The main enemy of plank roads is time and rot. Heavy use over a short period like a siege will be fine. Cutting down the forest smells a little bit like Sauruman's orcs, which might work for your fantasy. Also as regards cutting wood in service of a siege, the LORD GOD has some practical advice in Deuteronomy 20:20

Only the trees which thou knowest that they be not trees for meat, thou shalt destroy and cut them down; and thou shalt build bulwarks against the city that maketh war with thee, until it be subdued.

After the road and the siege, the forest will have only fruit trees.

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  • $\begingroup$ Good frame challenge! For what it worths, plank roads were even built well into the 20th century: There are still remains of the Old Plank Road in California. $\endgroup$
    – elemtilas
    Commented Jun 11, 2020 at 15:45

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