There are two things that need to be adressed:
1. Sound Isolation.
Other than increasing the background noise with a more subtle sound of some sort (white noise generators use averaging a work best when the sound source is consistent), your not going to be able to stop the noise from the street until you close the windows.
Do you have AC ? To be honest, if steet noise is that close to you, having the window open may allow alot of pollutants/dust to enter the home. Perhaps you should leave the windows open in the day while your out to keep the place air'd out, then use the AC in the evenings when your home trying to sleep/watch t.v.
2. Sound Absorbtion.
Drapes, carpet, soft furniture, etc. will help 'absorb' some of the errant sound from the street. Basically, these items help minimize sound reflections, which if received less than 40ms or so apart at your ear (reverb), can actually amplify the sound your hearing from the street.