Travelers

 Sure, let's expand ring by ring. So first, let's ask why someone would want to leave an arcology and step into a new one, so let's examine the people who would 1) Politicians, Execs, the Wealthy. Obviously their careers and duties take them all over the city, even the world. And even when the need isn't there, the drive and resources will be? Why go abroad? Because you can. 2) Emergency and civil services. As we established, while law enforcement and health care is provided locally, some crimes too big, some illnesses too serious, people need to travel to and from. 3) Not everything can be produced in the arcology. Luxury items, rare or geographically exclusive goods, items that need more overhead than the enclosed structure of an arco can be produced. 4) Competition and entertainment. Athletes need to play other athletes, musicians need to play concerts, models are not going to do the same runways and shoots for the same places. In fact, even, maybe especially, the amateur and low level individuals have to grind, riding the rails for the open mic circuit. 5) Just plain social butterflies, I mentioned girls trying to get into the skydeck, the real love of the game might travel from arco to arco. 6) Experts. Scientists might travel to labs, lecture tours etc. etc. Who else would the commuting/mobile life be set for?