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.

Also, birds, bees, and butterflies love hollies. We've planted at least 20 Nellie's, 5 needlepoints, and many dwarf burfords, that just keep on growing all around our lot which is on a busy road. Plus many pines, white and loblolly. Many large azaleas.

This is a VERY old thread!! And with quite a bit of misleading info as well. Plants cannot eliminate road noise - only a solid (and tall) wall can do that. They can reduce noise to some degree but only if very densely and deeply planted. The recommendation is a 50 to 100 foot deep planting of mostly tall growing evergreen species can reduce decible levels by 5-8 dBA.

What to plant to help (not solve) this issue depends on location and the site. Green Giant arborvitae, Leyland cypress or any fast growing native conifers are a good place to start but adding lower growing broadleaved evergreens like rhodies or hollies - plants that start branching and leafing out at ground level - will also help with noise abatement.