Reducing traffic noise in front and back yard
If you go with trees, go with spruce:  This gives you high density twig/leaf structure.  Evergreen leaf trees are also possibles.

Trees don't reduce sound by a lot, factor of 2 to 4.  That sounds like a lot, but it's only 3 to 6 db.  3 db is just detectable.  An earth berm is the best system.  Make it fairly steep on the side facing the noise so that sound is reflected up.  However, trees generate their own noise -- closer to white noise without the swoosh of traffic.  This will help mask the freeway noise.

Material for a berm can come from creating a pond.  A track hoe here in Canada runs about 1500 bucks a day, and a day's work moves a LOT of earth.

If you are going to build a wall, it needs to be massive.  Stone, concrete, brick, block.  Ideally such a wall has a hollow core, with the area density different on either side of the core. This reduces coupling between the front wall and the back wall.