The use of detailed pay item unit costs for estimating the cost of transportation facilities or services is straightforward but laborious. For a project, there can be several hundreds, sometimes thousands, of pay items that are priced separately. This costing approach typically forms the basis for contract bidding. The first step is the decomposition of a specific work activity (such as rail track installation) into constituent pay items expressed in terms of finished products (such as one linear foot of finished rail guideway) or in terms of specific quantities of material (such as aggregates, concrete, steel beams, formwork), equipment, and labor needed to produce one linear foot of finished guideway. After the various components of the work activity have been identified, a unit price is assigned to them (on the basis of updated historical contract averages or using the engineer’s estimates), and the total cost of the work activity is determined by summing up the cost of its constituent pay items. The level of detail of the pay items generally depends on the stage of the transportation project development process at which the cost estimate is being prepared. At the early planning stages, relatively little is known about the prospective design; therefore, the level of identifying the pay items and their costing is quite coarse (Wohl and Hendrickson, 1984). Cost estimators typically refer to four distinct levels of coarseness that reflect the stages at which such estimates are typically required.