Can a person be a symbol? It can if it's Annie's thoroughly weird brother, Duane. We don't spend a lot of time with him, but the time we do spend is important, and here's why: Just like Alvy, Duane has fantasies.
More specifically, Duane has visions of steering his car into oncoming traffic in a fiery, glass-filled explosion:
DUANE: Can I confess something? I tell you this because, as an artist, I think you'll understand. Sometimes, when I'm driving on the road at night I see two headlights coming toward me. Fast. I have this sudden impulse to turn the wheel quickly, head-on into the oncoming car. I can anticipate the explosion. The sound of shattering glass. The... flames rising out of the flowing gasoline.
Later, Duane gives Alvy and Annie a lift to the airport, and Alvy is thoroughly freaked out. We would be, too, and not just because Duane's played by Christopher Walken.
But there's more to Duane and his dreams than just being creepy. Film critic Tim Dirks argues that Alvy and Annie's treacherous ride with Duane "metaphorically foreshadows the crashing future of their relationship." As they speed across the wet blacktop, Alvy's mega-anxious.
Annie? Not so much. Similarly, when their romance does go up in flames, Alvy's a mess. He flies to Los Angeles, rents a car, and chases down Annie. Flying, L.A., driving: These are all things that stress Alvy out, big-time. When he meets Annie, he's stressed and petulantly asks her to marry him, twice. Annie, on the other hand, is as cool as a cucumber—just like she was in the car with Duane. She's unfazed by Alvy's concerns and demands.
Duane and his unsettling car crash fantasies are a symbol of not only the end of Alvy and Annie's relationship, but also of how both parties will handle it. Alvy travels to Los Angeles not only to blow up his relationship with Annie, but also to literally crash his rental car.