Generally speaking, as long as you aren't in an alley way or a driveway between two houses fairly close together, you should be fine. Radio waves bouncing off of buildings that are close together can throw readings off.
But you can get a good SWR and tune the antenna just fine while parked on the street in front of your house.
Radio traffic shouldn't make a difference since you are measuring reflectivity while you are transmitting. You won't be receiving any signal while your microphone is keyed. So, as long as you are set to read SWR, you should be good.