Boston, Massachusetts had a world-class traffic problem, an elevated six-lane highway called the Central Artery that ran through the center of down town and was one of the most congested highways in the United States. The accident rate on the deteriorating highway was four times the national average for urban interstates. The solution, called the Central Artery/Tunnel Project (CA/T), replaced the six-lane elevated highway with an eight-to-ten-lane underground expressway that culminates at its northern limit in a 14-lane, two-bridge crossing of the Charles River. Design of the interchange and highway approaches to the new Leonard P. Zakim Bunker Hill Bridge over the Charles River provided a northern “gateway” to the CA/T project. This interchange collects and distributes traffic on US1 from the northern suburbs and adjacent communities, to the Central Artery and Storrow Drive via the new Charles River crossings. The new structure carries over 200,000 vehicles per day. The total effort resulted in five construction contracts that included temporary bridges; reconstructed viaducts; maintenance and utility buildings; relocation, design and protection of numerous utilities; an open-transition roadway section; demolition of existing viaduct and buildings; urban architecture; and mechanical, electrical and fire protection systems for roadways. Complete engi¬neering services were also provided for the southern “gateway” that connects the Southeast Expressway with the Central Artery. The interchange is located in an area of restricted right-of-way with major utilities, considerable traffic conflicts, poor and contaminated soil conditions, and a sensitive urban setting.