You’re Already on NantucketPERIODICALLY AN IDEA COMES UP IN ONE town meeting or the other: a scheme to control the number of automobiles coming onto the island. Nothing changes, however. Even limits that seem sensible, say, two or three cars per household, never get implemented. As cars rule America, cars rule Nantucket. The summer traffic in town is so intense it has affected the way the island has grown. More and more facilities, from the supermarket to an ancillary post office, are located in what is now the outskirts of town. Small clusters of retailers, most of them pretty fancy, seem to have sprung up everywhere. Niche restaurants abound, every one with a parking lot. Downyflake Doughnuts, where working people and families can get lunch at reasonable prices, is out of town. (A hallowed institution, by the way, and well worth a visit, even at six-thirty in the morning.)I am on the island as I write this. I went into town at 11:30 this morning with a few errands to do, using a system that works well enough to allow me to do everything in two and a half hours.First you must plan.