Taking mass transportation is better for the environment, and it will reduce what you spend on gas. But even more important here, it's a great place to find stories.
On a subway or train or bus, keep an ear out for interesting conversations, or try your hand at starting one. If the crowd seems too preoccupied or unsociable for conversation, just observe. Find unusual details of dress, body language, etc. that you can use in a future story.
It has often been remarked that no plot is unique - in short that every story has been told before. But if you make your story detailed and specific, it will be an old plot told in a way that no one but you could have told it.
Variation: Walk. Keep an eye on the people around you and how they interact. Do they smile at you or avoid eye contact? Do you move out of their way or vice versa? If there aren't many or any people around, look for details in the environment instead. For more info on the benefits of walking, check out the 10,000 Steps Program.