For this you need to have unlocked the transfender modding service. First find a car that you are willing to give up and can be modded by transfender. Take it to transfender go to the modding screen the press triangle to escape. Then before you do anything get out of the car and leave it there. There will be no traffic but if you get into a car traffic will come back, so to make it so you can get into a car you will need to kill yourself. Then you can get in and out of cars and there will be no traffic.To turn traffic back on simply take your car to transfender goto the modding screen escape the drive your car out.