Increasing the density of your city's buildings is important to growing your population. Denser buildings support more Sims, whether as residents or workers.
The bars on each building in this view show the building's expansion progress. Buildings ready to expand will have a green bar.
Building won't be able to expand their density if the roads they are built along cannot support it.
Different Streets support the three building densities, but with four or six lanes, the wider Avenues can support more traffic.
You can upgrade roads after drawing them, but you cannot upgrade Streets to Avenues.
Be sure to use the right roads to provide room for growth. If you want skyscrapers, you need high density!