Last week's answer

Question: Spanning 5 miles over the Straits of Mackinac, the Mackinac Bridge opened in 1957 and was specifically engineered to avoid the tragic fate of the Tacoma Narrows Bridge, which tore itself apart in high winds. To ensure the bridge could handle severe Great Lakes gales exceeding 150 miles per hour, what unique structural design detail did engineers hide right in the center lanes of the suspension span?

Answer: Open steel grid decking on the two center lanes that allows high-velocity wind to blow harmlessly through the bridge deck rather than lifting it like an airplane wing.

The inside two lanes utilized open steel grates specifically placed to allow air flow. Combined with the structure's heavy stiffening truss, this feature prevents dangerous oscillations of the deck caused from wind. 

Sources: Historic Bridges