Roads don't just emanate out of nowhere either. They must be built and maintained, or they become useless. I doubt building new highways really takes much less time than building new railways.
We wouldn't need to build new highways for this. We'd just be changing how we use existing ones (replacing existing road traffic with more efficient road traffic).
Which will wear down bridges and streets far faster. There have been experiments with extra long trucks in Germany, but so far they haven't resulted in extended regulations. Longer trucks have severe limits of manouverability in small cities.
