Anyone Here Worked on Large Infrastructure Estimates?

I’ve been trying to understand how professionals manage cost planning for big projects like highways and overpasses. What kind of methods do experts usually rely on to keep estimates accurate and avoid unexpected overruns?

I’ve seen many teams using specialized resources, including bridges estimating services in USA, to improve clarity in complex structures and material breakdowns. But I’m still wondering how much of a difference these external services really make compared to internal planning.

Would appreciate any experience or insights from those who’ve handled similar projects.