<P> The bridge officially achieved its 100 millionth crossing exactly forty years after its dedication, on June 25, 1998 . The 50th anniversary of the bridge's opening was celebrated on November 1, 2007 in a ceremony hosted by the Mackinac Bridge Authority at the viewing park adjacent to the St. Ignace causeway . </P> <P> The design of the Mackinac Bridge was directly influenced by the lessons from the first Tacoma Narrows Bridge, which failed in 1940 because of its instability in high winds . Three years after that disaster, Steinman had published a theoretical analysis of suspension - bridge stability problems, which recommended that future bridge designs include deep stiffening trusses to support the bridge deck and an open - grid roadway to reduce its wind resistance . Both of these features were incorporated into the design of the Mackinac Bridge . The stiffening truss is open to reduce wind resistance . The road deck is shaped as an airfoil to provide lift in a cross wind, and the center two lanes are open grid to allow vertical (upward) air flow, which fairly precisely cancels the lift, making the roadway stable in design in winds of up to 150 miles per hour (240 km / h). </P> <P> The Mackinac Bridge is currently a toll bridge on Interstate 75 (I - 75). Prior to the coming of I - 75, the bridge carried US Highway 27 (US 27). It is one of only three segments of I - 75 that are tolled, the others being the American half of the International Bridge near Sault Ste . Marie, Michigan, and Alligator Alley in Florida . The current toll is $4.00 for automobiles and $5.00 per axle for trucks . The Mackinac Bridge Authority raised the toll in 2007 to fund a $300 million renovation program, which would include completely replacing the bridge deck . </P> <P> Every Labor Day, the bridge is open to walkers for the Mackinac Bridge Walk . </P>

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