Q: Why are there Interstate Highways on islands?
I'm not sure if anyone knows the reasoning behind interstate highways that do not connect to any other states, especially when located in a commonwealth. . .
However, the San Juan area boasts two interstates, I-? that runs north-south and I-? that runs east-west. They converge in the industrial and commercial core for two miles--one of the reasons San Juan is plagued by stop-and-go traffic.
Because the old core is located on the end of an island, there is a very narrow transportation corridor from Old San Juan to it's hinterland in the modern zone of Rio Piedras.