Native species to Maryland
What are the best species trees to plant that are evergreen and provide the best sound buffering from road noise?
Montgomery County Maryland
Consider planting a mix of different tree species (evergreen and deciduous) so you do not lose the whole stand to an insect or disease issue. You will have to look at the site sun versus shade and the mature height and width of the plants.
Depending upon your location and spacing you can stagger about two rows of evergreens of different species (if you have the space) and plant some deciduous flowering native trees in front of the rows such as witchhazel, redbud, serviceberry, etc.
You may want to contact some landscapers for more recommendations and design based on the site.
Here is a good reference and resources for native plants. See the mature height, etc. Consider American holly, eastern red cedar, sweetbay magnolia, thuja occidentalis, chamaecyparis thyoides, eastern white pine
non-native - juniper sp, cryptomeria japonica
Here are some links that may be helpful http://articles.extension.org/pages/62185/tips-for-using-plants-to-reduce-noise-in-the-landscape
Use for ideas only. Plant list may contain plants not recommended and grow in this area.