Except it’s more complicated than that. You can’t just scale the whole tunnel down to match your desired width because vehicles also have a height. A semi truck, presuming you want to able to move freight through the tunnel, is about 15 feet high. Add an extra clearance of 3 feet so they don’t hit the tunnel roof for a total clearance of about 18 feet.

The good news is the road surface doesn’t go through the centre of the tunnel, it’s somewhat lower than that. So for a single lane tunnel with a 12 foot wide road surface to maintain an 18 foot clearance you’d need an approximately 22 foot (21.62 to be exact) wide tunnel with the road surface a about 2 feet from the lowest point of the tunnel. But for two 12 foot wide lanes with the same clearance you’d only need a 30 foot wide tunnel with the road surface 6 feet above the lowest point of the tunnel.

In this case, the combined cross-sectional area of two tunnels that can carry one semi truck each is 734 square feet, but the cross-sectional area of one tunnel that can carry two semis is 707 square feet, or a difference of about 4%.