People who want to build roads can take out options on the property they want to buy--they pay the owner a small amount of money, well in advance, for the option to buy the land at a set price. If they do this over a wide enough area, chances are good they will have a continuous route. If not, they haven't spent nearly the money the would have spent if they chose to do it the silly way you seem to imagine they'd choose to use. But if so, they then exercise the options that they want simultaneously. No holdouts because people agreed to the sale long before.