On this subreddit and elsewhere, I have read extensive descriptions of how Roman roads were constructed (unfortunately, I do not have a link immediately at hand, a quick search didn't find a good one, and I don't have enough time to really hunt around for a good link -- so those who want to read more will have to find it themselves). Briefly, it sounds as if a great deal of excavation would have taken place along a defined path, and then the newly-formed trench would have been intricately filled with a variety of layers consisting of different materials (mostly rock and rubble of different types and sizes). This sounds -- though correctly me if I am wrong -- as if it was an extremely labor-intensive process.
The importance of roads to Rome's success seems to be so well known that it is almost an aphorism in our society. But when I really think about it, I wonder what benefits such elaborately-constructed finished roads really would have conferred over a simpler, more rudimentary road or path. For example, it seems to me like the major advantages of a road are ease of navigation and facilitating the rapid movement of people and goods. I would think that a simpler road or path (such as a well-trod dirt road) should definitely provide the same navigational benefits as a finished, paved road. And while I imagine that people, horses, and most importantly wheeled vehicles can probably move somewhat faster along a paved road than an unpaved road, I wonder how much of a difference this really would have made. Can an army really march or drag its wagon train materially faster over stone than over hard-compacted earth?
The only really major difference I can think of is that a stone road can be well-drained and will not turn into mud. How serious and frequent of a problem was bad weather hampering travel along less well-constructed routes in ancient times? Would all-weather roads have been a constant and persistent advantage over rougher roads, or would they have been something more akin to "road insurance" that meant no army would ever be preventing from moving by a storm at a critical moment? And if this is indeed the major benefit of finished Roman roads, did they really need to be so elaborately constructed (if I recall correctly it sounded like the various rocks and rubble went several meters deep) rather than just a half meter or so of gravel laid on top of a dirt trail (perhaps with some larger paving on top to facilitate the movement of wheeled vehicles)?
Are there other advantages to Roman roads that I am not considering? Would troops and goods have really had a materially easier time moving about in good weather as well as poor weather? And, as a bonus question, why didn't the Romans construct their roads out of concrete? It sounds like that would have been a simpler way to put them together than such a detailed and elaborate process of sequential stone-laying, though perhaps I am misunderstanding the difficulties of ancient concrete production.
Not having the roads turn into mud is really really really important. As an example, during the Crimean War of 1853-56 although on paper the Russians had an army of of 1.25 or so million men they could never use it, only ever deploying around 750k at a time. Part of the problem was that they din't have the equipment but the major bit was that they could not supply it since there were no proper roads, during spring and autumn they turned into bogs which were impossible to get wagons over. In summer they were dust bowls and winter they froze solid and were still incredibly difficult to transport stuff over. Indeed, it was quicker for the emperor in Petersburg to learn the events of the war with telegrams from Paris, it was simply faster then getting a message overland from Azov or wherever the battle. Now, this example is very far removed from the era that we are talking about, but still seems relevant considering the poor (nearly nonexistent) Russian transport infrastructure. For instance the Alamanni invaded Italy in 271, the emperor was far away in Pannonia, yet due to the extensive road network he was able to reach and defeat them relatively quickly, they crossed the mountains in early January and were routed very quickly later that year, unfortunately I can't find a date for the battle itself. This would have been impossible without the strategic mobility that an advanced road network provided. Another pro of roads that you seem to have overlooked somewhat is that the roads lubricate the flow of goods through the empire, especially bulk goods like grain, which was often imported to inland cities from Egypt, oil and wine -- their use was not purely military but was still essential. Further, the roads were much less expensive that one might think because the people that built them were legionaries when there was no campaign, they were professional soldiers and were still paid, it made sense to keep them busy when there was no fighting to do. Basically, Roman Roads really did justify their expense, mobility is one of the most valuable assets that an army can have the roads provided that in spades.
As for why concrete was not used I cannot answer the question, you'd have to ask an engineer. My assumption would be that they did not ave the technology to mass produce enough concrete and then move it the long distances that road making requires.
Incidentally the reason for the layers was to ensure proper drainage, if you just spread gravel the water cannot percolate through clay soil and you end up with another mud bowl.
Hey, I am gonna hop into this question first, "Are there other advantages to Roman roads that I am not considering?"
One of the reasons that I decided to go to college to study the Roman Empire was because I couldn't wrap my head around how they became so powerful. People love to attribute the rise and fall of the empire (and preceding period of royalty) to a variety of different things, but to me it appears that it was seemingly minor infrastructure innovations/changes that made a huge difference. One of these infrastructure things was their roadways.
It wasn't just the durability of the roads that changed things, but what took place along the roads. The roads gave them an accurate way to map the world. Something as simple as being able to measure a mile seems so easy to us currently, but back then it was an incredibly complex task. Roadways allowed them to -very- accurately map their own world which was a huge game changer when it came to moving armies as well as moving goods.
Saying something like “They are three days of riding away” spread across an entire continent was a logistical nightmare and something that plagued quite a few larger empires. (Many empires came up with some solutions, but Rome is the topic so I'll stick with that). Rome's roads changed that. A consul could tell when a cohort would arrive in Brundisium after leaving Rome within an hour or two.
So what else did they do? They had mile markers which were actually invented by the average “pace” of a soldier. They had inns/forts/outposts that made it easy to move information very quickly with fresh horses (precursor to the pony express). They had patrols that kept the roads relatively safe for the time, and this meant that goods had less overhead and would be more profitable to bring into the empire where they could be taxed.
As for concrete: It was used, but sparingly on the roads. The concrete of the period was much different than what we use today. It was used quite extensively at different points in the empire, but its durability and expense would make it impossible to use over 60k miles of far-flung roads. Multiple layers used for compacting and drainage were very very effective and efficient.
Hope that helped answer your question. The short answer, their roadways helped further the art/tool of topography which made the empire manageable. I would postulate that it was the “accuracy” of their roads that really changed things.
And remember, all roads lead to Rome.
Are there other advantages to Roman roads that I am not considering?
Here's one: propaganda.
Rome's roads were one of the most visible symbols of its power, especially out in the provinces. Troops come and go. But the road is always there, a reminder that the might of the Empire can come marching to your doorstep whenever it pleases.
The Romans' obsession with building straight, paved roads even when they had to construct multiple bridges to do so, had marginal practical gains (relative to the cost of construction) compared to more winding tracks, but it certainly helped the propaganda.
(Though we should bear in mind that not all Roman roads were -that- straight and big and high-quality. There were rules for minimum width and the like, so that carts could pass one another, but in practice these were sometimes flouted in difficult and mountainous terrain. There were limits to even Roman engineering.)
On a smaller scale, many of the early roads were constructed by individual rich Romans and bore their names. (Via Appia, Via Flaminia, etc.) Obviously, this was an immense source of prestige for both these individuals and their descendants. It was another motivation to make sure the roads looked impressive.
The practical reasons the others have mentioned in this thread trumped the propaganda and prestige function, but it's an interesting angle to look from.