Why do prices on public works projects often inch higher and higher?

The price tag for big public works projects often balloons higher than originally planned — and that often sparks public outrage.

The Washington Metro system’s Silver Line, the Columbia Pike streetcar in Arlington County, Va., and the Intercounty Connector in Maryland are three major transportation projects in the D.C. area whose costs have gone much higher than initially projected. A story in Thursday’s Washington Post looks at the three projects and how their costs, and those of other major public works projects, often snowball.