Mike,
Have you considered the evergreen Yaupon Holly? It gets 15 to 20' tall, and about 10 to 15' across. It does well in sun or shade and the females have decorative red berries that birds like. When you buy a Yaupon, make sure that you get mostly females (buy them with the berries on them) and mix in at least one male (for obvious reasons).
There are also Eldarica pines (also known as Afghan pines) which are relatively fast growing. They get really big--40-50' tall and 20-30' wide. But they'd look so fantastic with the snow on them!
Personally, I'd plant a mixed border--not all the same tree or all the same height. It gives a better visual effect, and if something dies out, you don't have a big evident gap. Good luck with your landscaping!

Those are good suggestions. Wax myrtle and Texas mountain laurel are both evergreen. Kidneywood is a fragrant, interesting small tree, but it's not much to look at in winter. Pond cypress is also beautiful, but just a stick in winter. What about a gingko? Argentine senna, if you can find it, is gorgeous and wouldn't overwhelm a yard that size. Can you tell I like the natives? Eve's Necklace is another one we really like that doesn't get too terribly big.

I second the Yaupon Holly and Wax Myrtle suggestions -- beautiful small trees that require just about zero maintenance.

Carla

Myrtle. I'd also put a cross vine on the fence. I'd also put a windmill palm in the corner.

We also back up to a main road, although we've got a bit of a green belt in between to help with some of the noise. We planted a row of Sandankwa Viburnum along the back to cover up our ugly privacy fence and to create a sound barrier. They are evergreen and dense, which helps with the noise. Ours are sort of in the background with other trees and shrubs in front, but they're actually quite pretty with sweet-smelling flowers in the spring.

Chrissy

Thanks for the feedback so far... I'll check into some of these.

cseim: Thanks for the info on the shrub option, as I've been interested in this too. Those get 6-8' wide AND deep though? That will really take up quite a bit of space. I assume I can trim them, but is keeping something that large trimmed back time consuming, etc?

Ours aren't that big yet, but I guess they will get pretty rounded. We put ours in the ground last year and they just really started taking off this spring. Maybe if you like them you could just use them in the corners of your yard to save space.

You might consider some roses also.

If you're inclined, you might plant larger, taller blocking plants on the OTHER SIDE of the fence and save your yard for more decorative, fragrant and beneficial plants. Normally I would NEVER recommend red tip photinia for anything ever, but if you put some on the other side they might go nuts and make a thick, bushy tall hedge to block the traffic noise and pollution. Or you could put our native cedars there - called mountain cedar, I think.

If you plant on the other side, you may need to use a sprayer to water them at first.

Here is a link that might be useful: Mountain cedar