A couple of Google hits on this chronicle have come up positive for "Marion County" and "Street name system". Marion County, Oregon is where I came into this world, so I spent a good deal of time looking at and wondering about addresses where I was growing up, so, while I don't have any reasons why things are the way they are there, I have many observations.
Here they are.
In Marion County (and adjoining Polk County on the west), all addresses radiate out from the center of Salem. Specfically this origin can be taken where State Street meets the Willamette River. The West Salem (and by extension, Polk County) area have addresses based on the way West Salem began, as a city in its own right, and whose logical origin is geographically quite close (literally, just across the river) from the east side origin.
So, for Polk-side addresses, the correspondence is not precise as you cross the river, but it is close enough that for the Salem westsider, addresses all decrease as you move toward the center of Salem as well.
Perhaps due to the way the city grew-somewhat governed by local geography, and without the huge development spurt that resulted in the gridiron-regular march of streets and avenues in eastern Portland. Salem streetscape has many different textures. The original city grids, downtown, are famous for 99-foot wide streets (necessitated, legend says, by the amount of space needed for a ox-team to execute a u-turn, reflecting Salem's history as a local break point for agricultural shipping).
Look at a good map of Salem, though, and it's the regularity and size of the blocks, in the center defined by the Willamette, Mission Street, 12th Street, and Union/Division Street, that jump out. They're big. They work out to around 500 feet square (compare with Portland's 200 foot square), and, with the street widths, the distance between street centerlines is very close to 1/10th of a mile.
Adjacent to the central core is a mantle of secondary development, which probably happened about the same time such areas as Laurelhurst and Ladd's Addition and the middle and outer eastside Portland areas were born. They reflect a desire to orient to cardinal points, perhaps for the convenience of surveying, and the relatively flat character of the adjacent area. The standard 10-per mile block pattern breaks down pretty quickly outside of the center, the blocks shrink, street spacing becomes uneven. Numbered streets, parallel to the river, don't start until you leave the central core, with 12th Street.
A third ring of development, dating from roughly the 1960s-70s to the present day, fill in the rest of the area out to the Urban Growth Boundary. These are the closest thing Salem has to suburbs, traditional tracts, split between the city of Salem, the city of Keizer, and still-unannexed county land.
The south Salem area (usually just called "out South" when I was growing up) seems apart, in a way. The main route out from center city is Commercial Street, which goes through a gridiron neighborhood as it climbs the hill but as it gets to the top, Commercial Street breaks east, Liberty Road breaks west, and the regular pattern gives out entirely. Commercial and Liberty forms a wishbone-like backbone to growth whilst cross-streets (Browning, Madrona, Vista, Boone Road, and latterly, Kuebler Blvd) provide access to the in-between area.
Perhaps due to this dendritic and irregular spread from the center, Salem's address grid isn't regular and predictable, as Portland's is. Altogether, the Willamette, State Street, Commercial Street SE River Road North, and Liberty Road South divide Salem into five directional areas, from the top going clockwise:
North: between the Willamette and River Road N. Suffix N.
Northeast: East of River Road N and the River, north of State Street. Suffix NE.
Southeast: East of a line formed by the Willamette River, Commercial Street SE, Liberty Road South, and (if you go far enough out) Sydney Road S. Suffix SE.
Northwest: The area on the right bank of the Willamette, commonly referred to as West Salem. The river doglegs at downtown Salem forming a reverse-L, providing a southern limit as well as an eastern limit.
Commercial Street is SE while serving as the S-SE interface, and Liberty Road is S when in that role. River Road North is formed when Commercial and Liberty Streets NE merge at the north end of Salem but the North suffix begins (as one is going north) at a minor side street called Stark Street N, which is adjacent to where Broadway St NE ends at River Road-just north of the North Salem Fred Meyer.
State Street is traditionally the hard boundary line betwen north and south Salem, and has traditionally carried no prefix or suffix, though logically would be State Street E. On recent trips to Salem I've spotted "State Street NE" on signs on the north side of the street, with "State Street SE" on the south side. I don't know if that's definitive-but I'd just as soon they'd leave it the way it was.
There is, strictly speaking, no Southwest. There is an area, south of Highway 22 (which forms an effective baseline after the river turns south at Eola Bend) that would suffice as SW, but the directional system has never applied out that far (or west of 55th Avenue NW) so, though the address pattern continues, the directional suffixes no longer apply as you move west into Polk County.
The format of Salem street names-directionals as suffixes-have given Salem addresses a particular and distinctive flavor. Long ago, before I became a permanent Portlander, I did a bit of business in Portland and, the vendor, noting my address, guessed correctly that I lived in Salem. All directionals in Salem and Greater Salem are suffixes: Commercial Street NE, 19th Street SE, 21st Ave S, Chemawa Rd N, Wallace Rd NW. The only other town in the valley with the same trend is Albany, which has actually had a compass-quarter directional system for years but which has only had painfully gradual acceptance and public use.
After you leave the city, addresses come 10 standard blocks to the mile. That's why it seems to take so long to get anywhere out there; as you approach Silverton on Silverton Road NE, the addresses are only in the 12500 range. Compare that to Multnomah County, where one is already well into the 20000s by the time one gets out to Gresham, which is about as far from Portland as Silverton is from Salem.
There is a lot more country in Marion County than around these parts. That contributes to the feeling of the Long Drive from city to city.
Street names in the county area do follow a sort of pattern, though they are spread so far out it's not immediately apparent. From the State Street base, e-w streets conform to a very loose alphabetical pattern, going up from A north and south. NE Streets seem to be named after flora (with a few exceptions for local pioneer families) and nouns and proper names in the SE. Numbered avenues cross them, going n-s, in a pattern that is continued from the urban area, though numbereds in town are termed "streets". Though they are named differently, they are still an extension of the numbered sequence begun with 12th.