Especially on digging-up-the-surface-streets kinds of projects, a lot depends on why they were digging, and what they find once they're down there. It gets especially fun when bits of infrastructure that belong to more than one agency or utility that don't talk to each other very efficiently are in the same general place.
Sometimes also an emergency dig gets a temporary repaving job until it can be done "right" under a master schedule.
Of course, some agencies manage construction a lot better than others. A friend was recently remarking upon the difference between two projects on streets near his house that involved paving. The street that, for historical reasons, was and is a California state highway seemed to get finished remarkably faster and better than one that was owned and therefore repaved by the city.
Getting a large bolus of money, e.g., from the Federal government can also have a startling effect on the timetable. When somebody with big money wants a highway built or upgraded fast, you'd think the Starship Enterprise was up there beaming down asphalt. The downside, of course, is that withholding highway funds, or threatening to do so, is a favorite way the Feds make states comply with various locally or regionally unpalatable mandates (speed enforcement used to be a favorite).
Incentive bonuses are another means of increasing schedule or other performance parameters that I've seen used very successfully, given good management of course.
