As always Paul Rabinow’s French Modern is an extraordinary reminder of how transport infrastructures serve functions at once military and commercial – and in fact “war, commerce, and transit” (in Paul Nizan’s memorable phrase) cannot be prised apart. Here is Rabinow about Gallieni’s pacification of Indochina:
There were only the most casual asides about more standard ethnographic realities-such as that the Mans didn’t like villages-Gallieni’s interest was infrastructural and instrumental. In village after village, he covetously and proudly noted every new bridge and road built; the French were spinning a growing spider’s web of installations-and Gallieni was the spider.
Roads were the key; without them there could be no movement of troops, no commerce, and ultimately no society. Gallieni was adamant that posts be constructed in durable materials, to demonstrate that the French intended to remain permanently. He ordered a masonry blockhouse built on a high outcropping overlooking the Chinese border. The post served a triple function: to observe both sides of the border, to provide solid military security, and to function as a representation of France’s enduring presence. All of these measures fell within a coherent if rather limited conception of conquest, human motivation, and social organization. Describing a meeting with one of the pirate-rebel leaders, Gallieni argued that signing a peace treaty was in everyone’s self-interest, as it was good for commerce. Although Gallieni basically distrusted the “feudal” warlords, he was pleasantly surprised to meet a Chinese warlord with whom he could drink (mediocre) champagne and discuss the need for peace and markets. (p. 148)
Definitely useful to remember that the roads were military conduits were commercial routes. Those functions were never separable because the separation of economic and coercive force is just an abstract heuristic we impose today on imperial infrastructures.
I have written a little bit about the function of these roads in counterinsurgency as well – in an article which unfortunately is behind a paywall.