As the former Chief of the California Office of Noise Control, I can give you a few solid tips.

1. Do NOT use plants in an attempt to control noise. Studies have shown that a dense evergreen forest parallel to a roadway, 100 ' thick, with foliage all the way to the ground will reduce noise levels only about 3dB – barely noticeable. Plants may be useful in hiding the noise source and thereby provide some improvement in your level of annoyance, but they are impractical for actually reducing the sound level. Use greenery for its own sake; you will need a solid barrier to control the noise.

2. A perimeter outdoor noise barrier needs to be solid (no cracks or gaps), have a mass of about 3 pounds per square foot, and intercept the line-of-sight from the source to the receiver by a substantial amount. It also needs to wrap around the receiver to some extent. When the barrier just intercepts line-of-sight, there is a reduction in noise levels of about 5dB (clearly noticeable). Then, for each foot that the barrier extends above line-of-sight, there is a further reduction of about 1 dB per foot. The practical limit of any perimeter noise barrier to reduce noise is about 15dB.

3. You say you live in S California, which means there is a good chance you rely on natural ventilation for some, if not most, of the year. In order for the building shell to reduce outside noise, windows facing the noise source must be closed. A typical house facade, with windows closed will provide about 25dB of noise reduction. With windows open, even partially open by an inch or so, that noise reduction drops to about 12-15dB. It is a waste of money to upgrade the windows in a building if they are left open for ventilation. And if you do upgrade your windows, you should seek advice from an acoustical consultant. Windows that provide better thermal performance do not necessarily provide better acoustical performance.

4. You need to consider the geometry of the relationship of your house to the roadway. If you are elevated from the roadway, then a perimeter type barrier may be practical. Otherwise you may have to reduce the noise at the building facade – or a combination of the two. There are no magic solutions to these types of problems. It is easy to spend money on the wrong things and end up disappointed. Get good PROFESSIONAL advice. Don't trust the landscape designer or the window supplier. They simply do not understand the physics of noise control.