as far as i know, there are two options:
- the Road building ability is tied to mancer blood i think (not sure, could also be Legacy of Serrane) - so if you play the Capitar faction (or a custom faction with Mancer blood [or Legacy of Serrane?]) you get Road building on Pioneers for free and the road building trait for other units
- i think one of the paths for commander champions (administration or merchant i think) ends with the road building trait, so if you level up a champ all the way to this trait, he/she can then build roads.