The county hospital is located at the center of a square whose sides are 3 miles wide. If an accident occurs within this square, then the hospital sends out an ambulance. The road network is rectangular, so the travel distance from the hospital, whose coordinates are (0, 0), to the point (x, y) is |x| + |y|. If an accident occurs at a point that is uniformly distributed in the square, find the expected travel distance of the ambulance.