When bicycles were young, the world was connected enough for this. But train tracks were a better technology (with better cost/benefit) at the time.
> In the US some roads were built in the early days, but canals and later the railroad had enough advantages over road transport (both can haul far more bulk) that for most roads were not worth building and the dirt track was fine.
Yes, exactly!
> [...] but farmers (the majority of the population) didn't travel often enough that the cost (labor) to build a road was worth it - they just stayed on the farm when it rained.
And they also only just needed to get to the next train station. The didn't need a county [sic!] spanning network of roads, just many individual star topologies.
However, I am not sure that Rome mobilised an unusually large proportion of their population. For the longest time, Romans were better at warfare than most of their neighbours, but I'm not sure they actually did more of it.
Their neighbours weren't as organised, and not as organised on as large a scale; but they perhaps mobilised just as much?
(https://acoup.blog/2024/02/16/collections-phalanxs-twilight-...)
In the US some roads were built in the early days, but canals and later the railroad had enough advantages over road transport (both can haul far more bulk) that for most roads were not worth building and the dirt track was fine. Towns would build roads in town because mud was annoying enough, but farmers (the majority of the population) didn't travel often enough that the cost (labor) to build a road was worth it - they just stayed on the farm when it rained.