Upgrades
In 1958, a $14.5 million ($116.8 million in 2012 dollars)) upgrade created a southbound span and doubled the capacity of the bridge. The new bridge was built with a "humpback" that provides 72 ft (21.9 m) of vertical clearance and minimizes bridge openings. At the time the new bridge was opened, the old one was temporarily closed for rebuilding to give it a matching humpback section. When both bridges were first open concurrently, in 1960, each bridge became one-way, and tolls were reinstated at $0.20 for cars, $0.40 for light trucks, and $0.60 for heavy trucks and buses, until removed in 1966 after the construction expenses were paid off. A $3 million ($5.3 million in 2012 dollars) upgrade to the lift cables, expansion joints, and a deck repaving was completed in 1990. The diesel generator used to power the lift was replaced in 1995 at a cost of $150,000. In 1999, the bridge was repainted at a cost of $17 million. A $10.8 million electrical upgrade was completed in mid-May 2005.
The bridge is 3,538 feet (1,078 m) long with a main span of 531 feet (162 m). The vertical lift provides 176 feet (53.6 m) of river clearance when fully opened. Openings last about ten minutes and occur between 10 and 20 times per month.
Signals for several miles each direction warn of bridge opening since traffic has to stop and wait. Due to this interruption, the Interstate Bridge is one of the Federal Highway Administration's highest priorities for replacement. Commercial river traffic schedules passage to avoid rush hour.
In 2006 the six total lanes of the bridges carried 130,000 vehicles daily. Full traffic capacity occurs four hours every day.
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