Poplar would be a very poor choice as it spread by shooting new stems up from its roots.

Avoid trees that have large surface roots as they mature (many maples).

Planting trees is an act of faith in the future. I highly recommend it! If it were me, I'd plant the tunnel using 'generational' trees - i.e. long lived, majestic trees. I don't know what they might be in your zone... Space them based on their mature size - which is likely to be further apart than 20'. I'm a fan of the philosophy (paraphrased from someone-or-other...;-) 'Live like you're going to die tomorrow but plan (- or plant in this case...) like you are going to live forever!'

(This advice is coming to you from someone who has outlived their life expectancy by 5 years and counting...) My role model is William Robinson who was planting an orchard in his mid-90s - an optomist if there ever was one!

I'd agree that maples and sweet gums (Liquidambar) should probably be avoided as, while fast growing and offering great fall color, both are notorious for surface roots and disrupting pavement. I'd also discourage you from the flowering cherries -- they are quite problematic in the NW and are typically plagued with disease and pest issues. The maintenance to keep them healthy outweighs their all too short spring attraction.

I'd consider looking at an ash - Fraxinus oxycarpa 'Raywood' is a good choice but you could also choose our native ash (Fraxinus latifolia) or one of the other many species and cultivars available. Japanese Zelkova is a good candidate also - both of these tree species tend to grow rather rapidly yet not outgrow their welcome, if you know what I mean :-) If you don't mind bright gold foliage, Robinia pseudoacacia 'Frisia' is a stunning tree and can be a nice contrast to the overwhelming green we seem to have in the NW. All of the above are pretty darn low in the maintenance requirements.

There's lots of other possibilities - oaks, tupelo, birches, etc. but most will grow slower than you might prefer. And do follow the advice about spacing according to their mature spread - it may look a little sparse initially but will provide better results in the long run and create less maintenance as they grow.

ps. I'm not that far away - just outside of Poulsbo - and any of these suggestions should be relatively easy to find locally and well-suited to this area.

I'd suggest that you consider whether you want tree species for the immediate impact, or whether you are also truly concerned about the viability and costs to maintain these trees beyond the time span you have laid out. If you are willing to spend what it takes to plant at such close spacing and of initial size to have some immediate impact, it seems rather ridiculous to be complaining about the cost of a consultation to get advice on trees that best serve your purpose.

Further questions to be addressed would seem to include who will do the maintenance on these trees as you don't seem capable, and have you budgeted for that? It is also likely that you will need to provide irrigation for these trees, and that is not a small budget item either. None of these issues are likely to be addressed by a nursery that is simply interested in making the sale, and is reason enough to suggest that the free advice likely to be obtained from a nursery on such tree selection is not very well rounded.

While I personally like the Robinia pseudoacacia 'Frisia' as a beautiful, fast growing tree, it is also subject to weak branch angles and wind damage, and needs selective thinning and shaping to minimize broken limbs, similar to the problems that the equally fast growing Bradford Pear presents.

Planting so many trees presents issues of maintenance costs for the future caretaker of the property, be that you, your wife, your children, or a future owner after the property is sold. Resale value of the property could well be affected by making the wrong choice based on initial cheap cost of trees and fast growth, as well as saddling the future caretaker with excessive maintenance costs or the cost of removals because the trees fail.

If you are in zone 8, and you take the heritage view, you might use oaks of some kind for the plantation home look.

Might you consider shrubs that bloom or color up in your favorite season? Or even combine some stately-to-be trees with shrubs that will give impact right away and in a few years.

Or, forget the grow-overhead tunnel look and plant reasonably spaced dogwoods or crape myrtles that are beautiful much of the year.

poplar, quaking aspen, are a very short lived tree that won't last and will leave you with dead trees and stumps to be removing..

you want something with a high canopy that doesn't drop fruit or seeds that will be sprouting all over the place.

oaks are beautiful as frankie said, but also they drop nuts and take forever to grow.

myself..i would go to a good nursery and pick up some maples that are already about 8' tall or taller..and i would put them more like 25 feet to 30 feet apart..and stagger the ones on the opposite side of the drive so they fall between the ones on the other side..

there are other high canopy low maintenance trees, but pick those that don't drop a lot of nuts or seeds or leaves..

maple trees leaves can be mowed over in the fall with a mulching mower or picked up with a bagging mower and used to mulch your flower gardens..or if mulching mower..left to feed the lawn.