The means by which you move forward in your life, where traveling represents moving forward in time through your life.
Other people in the vehicle can represent individuals often around you in real life, or just people in the world in general. Being in a vehicle with other people can represent a recent or imagined shared situation, such as a dream about being on a bus with your project team from work can represent yesterday's team meeting.
For more clues to meaning, pay attention to the characteristics of the vehicle. For example, an expensive car might represent abundance or pretension, a damaged vehicle might mean you're feeling vulnerable or careless somehow in your life, and a dump truck might represent cleaning out your to-do list or toxic relationships.