Seeds can become stuck in tire treads or mud on the vehicle and be carried to unaffected areas.
If you find a small number of isolated noxious weeds that have no flowers or seeds, pull the weeds and leave them where you found them to dry out. If you find noxious weeds and they have flowers or seeds, pull them, and place them in a plastic bag or container to avoid spreading seeds, and either burn them or dispose of them in a sanitary landfill.