What incentive does a city/state have to expand or build roads?

Our area is growing in population very quickly and traffic is a concern since the buildings are popping up faster than the roads. The city is getting lots of money from the expansion, which is great for them, but it had me thinking: What exactly motivates a city to build the roads it needs in a growing area? What stops them from just saying, Frick it, I'll build something else, etc?

Seems like an obvious answer but I didn't want to think about it.