Trees do not stop noise, you need a solid fence to reflect noise. See the link below for a recent discussion.

Here is a link that might be useful: Noise Barrier

Trees do help with noise reduction.. Lots to think about. Cedars and blue spruce. Dont' plant them in a straight line, plant them in a zig zag design so that branches will grow wide and overlap the ones from the other trees.

We live next to a 2 lane road, not to traveled but still a problem as far as privacy. We built a berm (about 3-4 foot, with some natural hill/slopes to it) Then I planted a total of (16) 7- 8 foot pines/spruces. We have about 1 1/4 acre so I had a lot of room to cover. I included Austrian, Norway, and then Siberian. I also intermingled some tough shrubs and 3 Sunset Maples to make the sight more appealing. The arborist later told me that the Austrian's were not a good choice, disease prone. But the other's should be ok. Another thing he said was not to plant just one variety of tree, etc. because if there is a blight/disease to that type the whole thing is a goner.

I'd like to underline something "perkladd" said: Plant different species. I had a tall hedge (12') of Port Orford cedar which was about 50 years old. It started dying about 5 years ago. Apparently it is an epidemic in my area and there is nothing that can be done. I have been taking it out as it dies and have lost all my privacy. It did a pretty good job of noise reduction.

The large cup shaped leaves of Rhododendron and Hollies are considered the best noise reduction barrier. A note on the Rhodies if you do not let them flower for a few years they grow very fast and fill in better than if you allow them to flower.

I've heard hollies reduce pollution also.