This article from Reason argues that a double-decker traffic tunnel can effectively reduce congestion in Atlanta -- one of the most congested cities in America.
"In April, Forbes ranked the metro-Atlanta region the second most congested city in America. It has been estimated that the direct cost of that congestion to Georgia motorists is $1.75 billion per year. Fear not my fellow weary travelers, there are solutions out there that, by thinking outside the box, can address this looming crisis."
"On a recent trip to the metro area, I learned that Cobb County has broken ground on a 24-foot diameter wastewater tunnel that will be 29,100 feet (5.5 miles) long. This sewer tunnel will run from just south of Austell to the South Cobb Water Reclamation Facility just north of I-20. I was already aware of the Chattahoochee Tunnel that is 18 feet in diameter and 9.5 miles long, and it got me thinking about tunnels and creative solutions to the congestion crisis that Atlanta faces day in and day out."
"What if I were to tell you that there is a way to provide the needed capacity in the most congested area, the Downtown Connector, while protecting Atlanta's neighborhoods? Well, my colleague, Robert Poole has proposed building a double-decked road tunnel, (similar to the new sewer tunnel that is currently underway in Cobb County), which would link the southern terminus of Georgia 400 with I-20 and later with the northern terminus of I-675. This tunnel is only one of perhaps half a dozen major projects needed relieve metro Atlanta's congestion but would provide the most dramatic relief. This proposed tunnel would have an inside diameter or 45 feet and each deck would have three 11 foot lanes and an overhead clearance of 12 feet allowing the tunnel to accommodate buses as well as SUVs and cars. The northern tunnel would be 5 miles long and the southern one would be 3.1 miles in length."