Don't infrastructure projects scale well with regards to costs? I assumed that e.g. building a three lane highway isn't 50% more expensive than building two lan

Don't infrastructure projects scale well with regards to costs? I assumed that e.g. building a three lane highway isn't 50% more expensive than building two lanes, but maybe it even complicates things more as you run into more issues the more space you take up (can't squeeze through gaps, more attack surface for Godzilla and whatnot). Isn't infrastructure profiting from automation as much? Health & elderly care has severe limits there until we get to very advanced robots in a few generations, but infrastructure feels easier.