"Don't worry. He'll grow out of it." Well, I'm not worried. And I also don't think he'll grow out of it. Our son's been obsessed with vehicles since the age of 18 mos., when he got a board book about construction vehicles called "Diggers and Dumpers." His first non-needful word (i.e., after "mommy," "daddy," "blankie," and "paci") was "dumper." He's five-and-a-half now, and the obsession hasn't waned; it's just been refined.