These long duration projects always go up in cost due to inflation. It’s really not a big deal except if you want to write hyperbolic headline like this article

> Unit prices were established at the time of bidding but the contract does include provisions for adjustments associated with standard price indexes such as diesel fuel, asphalt, and steel

Some projects have major unexpected obstacles like the route interstate 99 when they realized they exposed acidic rock and had to spend $100 million in remediation