Caulk around doors and windows. Cracks in these areas allow sound to enter.
Replace your windows with soundproof double-pane windows. Keep your windows closed and cover with heavy curtains or other window coverings.
Replace any hollow doors with solid models.
Add more insulation in between walls or add an additional layer of drywall over the existing walls.
Add carpet in rooms with hardwood or tile floors. Add area rugs if carpeting is not an option.
Plant trees and install a privacy fence to absorb sounds outside. The more materials you have outside, the more sound will be absorbed.
Add white noise to your home to cover up the traffic sounds. Fans or air purifiers provide a soft, constant noise that distracts from the traffic.