Q:

How can I reduce street noise behind my house?

My house backs up to a street, with 3 lanes in both directions.
Even though there is a good 30 feet from the back fence to the road, the street noise can get pretty loud during peak traffic times.
I'm looking for a solution to reduce the noise in the backyard.
The fence is currently an old wood post type (5ft. in height) with gaps between it - so I am sure it is not doing anything to reduce noise.
Is there anything I can put on the fence to drastically reduce street noise?
Is there a different type of fence that does this really well - that you have tried?

A:

To provide even a little noise reduction, you need to eliminate air gaps in the fence.   If wind can blow through, sound can come through too.  There is little you can do to improve the fence without first sealing the gaps.   Adding foliage (i.e. shrubs or vines) will do nothing.
To seal the gaps you need to 
After sealing the gaps, the next steps would be to increase the barrier height and/or adding mass to the barrier (usually done by adding extra layers)
Here are some US govt documents related to barriers.

http://www.fhwa.dot.gov/environment/noise/noise_barriers/design_construction/keepdown.cfm
http://ntl.bts.gov/lib/35000/35700/35778/FHWA-EP-00-005.pdf

