This is an excellent question. Act One of Arthur Miller's Death of a Salesman begins with Willy Loman returning from one of this many business trips as a salesman. The difference with this particular trip is that Willy confesses that he nearly "hits a boy" with his car,and that he kept "going off onto the shoulder" while driving; that he just simply "could not make it."
Although Willy's wife, Linda, secretly knows that Willy has tried to kill himself in the past using his car, it is interesting that Miller allows the audience into Willy's state of mind. This is done to give the audience an instance of foreshadowing; establishing the perception that this incident is not isolated, and it will be either repeated, or that it will become part of a separate, or more complex, series of events.
The fact that Willy "could not make it" but "as far as a little above Yonkers" is a form of foreshadowing. That line alone provides the audience with a plethora of facts that are to be unfolded later on in the play. For once, it shows that Willy has been run down by work, as well as by the changing society; he is no longer the agile and active younger salesman who could take the long trips everywhere for the sake of selling.
Second, it foreshadows that, like his car- an old Studebaker, Willy just cannot and will not go any farther than his immediate limits. Equally he has not and will not go any further than his fantasies. He has not achieved the high status that he had hoped to achieve in the past, and he has really nothing left for his kids either. Like his car, he has "had his run", and he will not go any further. This is the reason behind Willy's odd and bizarre behavior: rather than exploding and taking his frustrations on everybody (although he does this in a few occasions), he implodes instead and slowly falls into a mild insane world full of memories, flashbacks, and what if's.
Therefore, Yonkers is merely an allegory to Willy's goals in life: just like he could not drive any further than Yonkers, neither could he achieve more than the bare minimum that he had wished that he could.