The argument is, if the traffic is just going to expand to fill the new roads we build, then why not build more densely instead and help people have shorter commutes. Also some arguments around vicious cycles of building more roads, leading to more traffic, leading to demand for more, bigger roads, etc, vs. supporting biking/walking/public transit
The problem with density is that it assumes that housing will be efficiently allocated so it reduces people's need to commute. Unfortunately there are a lot of other factors involved and density in some cases ends up making traffic worse.
The argument is, if the traffic is just going to expand to fill the new roads we build, then why not build more densely instead and help people have shorter commutes. Also some arguments around vicious cycles of building more roads, leading to more traffic, leading to demand for more, bigger roads, etc, vs. supporting biking/walking/public transit