Willy tells Linda that he is "tired to the death ... and couldn't make it." He claims he "couldn't drive any more. The car kept going off onto to the shoulder." Willy was afraid that he might have an accident. Linda fears he has smashed the car because his return is unusual, especially at this time of night.
Later Willy reveals to Happy that he "got an awful scare. Nearly hit a kid in Yonkers." He has begun not to trust himself behind the wheel of a car, and he shouts at Happy, "The woods are burning! I can't drive a car!" Driving a car is essential to his job as a salesman in New England.
Willy should be driving to his work since that is the only way he can actually travel, however, Willy had a feeling he would drive off the edge or hit someone so he came home. At first Linda is worried that he may have ruined the car, but this also foreshadows what is wrong with his mental health in the first place.