The average U.S. airport is approximately 40 years old. That makes them seriously outdated–which will come as no surprise if you've recently passed through those gates of hell, LaGuardia and JFK.

As a result, American airports are facing the necessity of an airport building boom. T.J. Shultz, president of the Airport Consultants Council, recently told Architectural Record that many of these aging airports face a choice: "Authorities and designers have done all they can to account for differing airline service trends and new technologies coming into play, and now they're in a position where they have to either refurbish or rebuild." Today, 50 U.S. airports have plans to invest an estimated $70 billion over the next three years to modernize their buildings.

It's a huge opportunity for cities to rethink transit–and for designers to help rethink the airport. Co.Design asked four architecture firms which airport they'd redesign and why. Here are their answers.