Oh man, sound abatement is something I've studied and experimented with for a long time. I'm also a long-time musician so containing my own noise, and that of others when recording, has always been something to work on.
First question is does the noise just bother you when you are outside, or both inside and outside? Inside the house you can make improvements that will have you not noticing that noise at all. Better windows, heavy curtains on windows, white noise (bubbling indoor fountains) all work well. Adding insulation to walls if the existing insulation is non-existent or poor - blown in cellulose will create a lot of sound deadening. Even changing your siding can make a huge difference.
If the noise is just when you are outside, you've got 4 options - deflection, diffusion, absorption, disguising.
Deflection is putting up physical structures that will reflect the noise back where it came from. This is most effective when you can put the deflecting structure closest to you. So if you have a great sitting area, create a sound bubble there by putting a tall solid screen directly behind it. If you can build a solid wall to deflect noise, even better. Even a wood fence will cut some decibels - again, the closer you are to the structure the quieter it will be. Tall vegetation - big trees, shrubs - will also do some minor deflection.
Diffusion - breaking up the sound waves. This can be done with fences, walls, big trees. If you can plant a mini forest between yourself and the source of the noise you'll be surprised how much sound can be diffused (have you ever hiked in a forest that was eerily quiet?). But you have to think thick here.
Absorption - this is soaking up the sound waves. Acoustical mats hanging from fences and walls work well for this. Dead air spaces also work well for this - the principal of building music studios leaves hollow spaces in walls where sound goes to die. There are some pretty impressive industrial machinery acoustical mats out these days that can drop decibel levels by 20-40 decibels, which is pretty significant. Trees and big shrubs will absorb some sound waves. Most physical structures will not unless they are pretty soft - a wood fence will absorb a little bit of sound but a concrete wall will absorb virtually nothing.
Disguising - this is creating a competing white noise, like a loud fountain outside. You can get fountains that are louder than any noise you're going to hear anywhere, though you don't usually need to go that extreme if you employ this with some of the other methods.
Also remember that there's a psychological component to all of this. The brain perceives sounds that it cannot determine the source as softer than those it can identify the source. So if you have a nice bubbling fountain and a little forest blocking out seeing anything far away, you will perceive a greater decibel drop than if you can see the highway.
