Most of the highway bridges that are in use today are most likely constructed in the 20th century with an average age of 45 to 50 years. Structurally deficient and functionally absolute bridges constitute about 10 to 20 percent some of them occasionally collapsing causing life and economical losses. Building sustainable bridges is a major task today while some historic bridges still stand still to the centuries. Kesikkopru Bridge located in Sivas province of Turkey, which was built in 1292, is an outstanding example to sustainable bridges. The stone arch bridge constructed in the Seljuk era is about 326 meters long with 19 arches and has been the focus of this paper. Measurement of structural dimensions in the field, truck loading analysis using nonlinear compression line analysis, ambient vibration testing, finite element modeling, 500 year return period flood water dynamic modeling, and earthquake response spectrum analysis have been conducted to assess the structural characteristics, response to natural disasters and traffic loads. The results showed that the bridge remains close to linear range during earthquakes and floods and has been overdesigned for vehicle loads. Simple hand calculations and typical dimension ratios obtained from the bridge provide hints for stone bridges being sustainable for over 7 centuries of life span. The authors debate building overdesign structures might better optimize the resources for sustainable bridges that require lower maintenance and have longer service life with higher level of structural safety. After all, building over safe bridges might also be more economical in the long run.