A bridge deterioration model is estimated from the National Bridge Inventory that explains and forecasts future condition as a function of bridge material, bridge design, operating rating classification, average daily traffic, the state where the bridge is located, and the age of the bridge. Over the 95-year analysis period, the rate of bridge deterioration with age is a third-order polynomial function. However, the relationship between condition and age is approximately linear until age 65. Holding all else constant, a bridge substructure in the Northern Plains loses approximately one-half of a condition rating point every 13 years until age 65.