When you're outside and alone there's no need or reason to wear a mask.
If you do have a virus such as Covid-19 or the flu or cold virus and sneeze or cough and nobody is around the virus will just leave your body and go through the air.
The virus will not survive long and eventually the virus will no longer be in the air to infect anyone else.
Although it's still a good idea to wash your hands often and use some hand sanitizer especially if you do sneeze or cough.
I would recommend having a face mask in your pocket or purse and have it available should you need it when someone comes up to you or you need to go into a store.
I always keep a face mask in my purse just in case I need it.
But when outside or in your own vehicle you do not need a face mask.