One of the great mistakes this country made decades ago was to seriously damage a large number of American cities by cutting freeways through existing neighborhoods. How much damage? A 2006 paper by Nathaniel Baum-Snow gives this answer (via Ryan Avent):
Between 1950 and 1990, the aggregate population of central cities in the United States declined by 17 percent despite population growth of 72 percent in metropolitan areas as a whole. This paper assesses the extent to which the construction of new limited access highways has contributed to central city population decline. Using planned portions of the interstate highway system as a source of exogenous variation, empirical estimates indicate that one new highway passing through a central city reduces its population by about 18 percent. Estimates imply that aggregate central city population would have grown by about 8 percent had the interstate highway system not been built.
I wouldn’t say that we shouldn’t have built highways—it’s more that we shouldn’t run highways smack through existing cities or made circumventing existing cities such a priority of the highway system. Connecting one metro area to another with limited access highways makes perfect sense—slicing a city into pieces with limited access highways does not. And when considering these estimates you need to recall that it wasn’t free to build these highways, you could have made alternative infrastructure investments in rail or buses or even just nicer-looking boulevards.