There are only two bridges across the Cape Cod Canal offering access to and from Cape Cod, the Sagamore Bridge and it's twin, the Bourne Bridge about a mile and a half south. The traffic backs up for miles on weekends, especially during the summer.
The Bourne Bridge connects traffic from Route 3 on the mainland to Route 6, the Mid-Cape Highway.
After the first bridge, completed in 1914 by a private individual proved inadequate and unprofitable, the Army Corps of Engineers was given the task of widening the canal and building new bridges. The canal widening project was a Work Progress Administration (WPA) project. The Corps completed a major renovation of the bridge in 1980, and still is charged with its maintenance.
The four-lane bridge carries 55,000 vehicles per day, though this is probably doubled during the summer and on weekends. There is pedestrian walkway as there is on the Bourne Bridge. Within the last few years the traffic rotary on the mainland side of the bridge was replaced with new overpass approaches, greatly reducing the congestion.
A bit of history from
Mt. Holyoke College
On March 31, 1928, Congress directed the U.S. Army Corps of Engineers to implement as series of improvements, including the replacement of the three bridges. The Corps selected two land areas that were naturally elevated in order to accommodate large sea going vessels. The vertical clearance was 135 feet, and a horizontal clearance of 480 feet. The existing railroad tracks made it impractical to relocate the railroad bridge. The Corps selected a vertical lift bridge design allowing for the same clearance as the highway bridges.
The Public Works Administration, the group responsible for managing Emergency Relief funds during the Great Depression, authorized the construction of the three bridges on September 6, 1933. The National Industrial Recovery Act of 1933 provided 4.6 million dollars in federal funding. In accordance with PWA regulations, work was distributed widely and employed about 700 workers.
The Sagamore and Bourne highway bridges that exist today were created simultaneously as work began in December 1933. The Sagamore Bridge was constructed about two and one half miles from the eastern end of the Canal. The Bourne Bridge was constructed about one mile from the western end of the Canal. The bridges have a horizontal span of 616 feet and a vertical clearance of 135 feet above water. They are designed for four lane highway traffic as the roadway width remains 40 feet. The total cost 3.4 million dollars.
From the U.S. Army Corps of Engineers
Spanning the original Cape Cod Canal, constructed by New York financier August Belmont, were two twin cantilever draw type highway bridges and a draw type bascule railroad bridge with openings over the water of only 140 feet. These narrow passages, combined with the swift currents and winding approaches, made mariners leery of using the waterway. Going around the tip of the cape, however, added up to 165 miles to their trip through waters where many ships had already sunk. Since the drawbridges could only be crossed when there was no marine traffic, automobiles going to and from Cape Cod also experienced delays.
When the Corps of Engineers was assigned responsibility for the waterway in 1928, a series of improvements began, including the replacement of the three bridges.
On September 6, 1933, the Public Works Administration (the agency responsible for managing Emergency Relief funds during the Great Depression) authorized construction of three bridges over the Canal. Contractors began laying the bridge foundations in December 1933. In accordance with Public Works Administration regulations, work was distributed widely; and, wherever practical, hand labor was used instead of machinery to provide as many jobs as possible.
The two highway bridges were designed and Fay, Spofford and Thorndike of Boston supervised construction. They retained the Boston architectural firm of Cram and Ferguson to advise upon architectural details and the appearance of the structures.
The Sagamore Bridge was constructed about two and one half miles from the eastern end of the Canal land cut, and the Bourne Bridge about one and two thirds miles from the western end of the land cut. The bridges each have a main span measuring 616 feet between centers of support and a vertical clearance of 135 feet above high water. The structures differ in the number of approach spans. The roadway width of the bridges, designed for four lane traffic, is 40 feet between curbs. Built simultaneously, the bridges were dedicated on June 22, 1935, and opened to traffic.
For more detailed information and history, here are two good resources.
Boston Roads