City's tend to have a lot of latent demand. There are some highway segments outside of city's that get minimal usage after they where built but which then grew over time instead of the sudden surge due to latent demand ex: 81 in southern VA which now sees heavy use, but was fairly empty right after construction.
However, in the city case new roads with high latent demand you may see a shorted rush hour which is a very useful side effect.
It seems to me that an urban car tunnel would be the most likely thing to induce demand: it's useless for local traffic, and is so expensive that it's only going to built in places with lots of latent demand.
However, in the city case new roads with high latent demand you may see a shorted rush hour which is a very useful side effect.