Miscanthus �Giganteus� would make a good barrier. I'd pick one of the varigated varieties because it's looks better from a distance. Like Miscanthus giganteus 'Lottum'
9' x 6'

As much as I love grasses, I probably wouldn't use one for your purpose. The only screen-sized grasses in your zone would be warm season varieties which have to be cut to the ground in early spring, and will not regain screen height until early summer at the earliest. That leaves you without a screen for quite a while. There are no evergreen screen-sized grasses which will live in your zone...other than that grass everyone forgets is a grass...Bamboo.

Check out Fargesia dracocephala 'Rufa.' It grows 7-10 feet tall, is not invasive and is evergreen. In a few years, you'll have a beautiful, nearly maintenance-free screen.

I forgot to mention it is full sun and relatively sandy soil.
Will the fargesia do ok up there? Where i am thinking of planting is actually on top of a 5 foot retaining wall. I can not build a Privacy fence more than 3 feet high in this area due to city ordinance.The rest of my property sits on the low side of that retaining wall. Also, how does bamboo do with salt spray in the winter. It will be planted about 18 feet from a very heavily traveled road. Thanks don and spider.

This specific Fargesia will handle full sun as long as you keep it irrigated, and sandy soil is fine. As far as salt spray from the highway, I doubt if it would get enough to harm it. 18' back and 5' up is a long way. If the salt and ice damage it, it would only be foliage, which would be replaced the following year.