Which states have clusters of similar-numbered state roads close to one another?

[TheHighwayMan3561]

Minnesota has MN 56, 57, and 58 (along with the old MM 59, which was absorbed into US 63) in the same general region. It came later into the area, but all three of those also intersect MN 60, and a piece of MN 55 could be considered in this area's orbit as well.

The highways in the 200's that do not go to state facilities look like they are, or were, clustered. The 240's go from the Twin Cities area down to Caledonia. The 250's go from Lanesboro to the Blue Earth area. The 260's go to the SW corner. The 270's appear to be north of the 260's.

I didn't even realize that. The 210s are in far northern Minnesota, 220s in north central, and the 230s in central Minnesota.

[US 89]

You'll find quite a few clusters scattered around Utah. The original 1927 system was highly clustered by geography - route numbers in the teens were all in the southwest part of the state, 20s in west-central Utah, 30s in northern Utah, and 40s generally in eastern Utah. And as the state assigned more highways, similar numbers would usually be placed near each other - for example, the original SRs 103 through 110 were all east-west routes branching off US 91 in Weber and Davis County, with numbers increasing to the south.

Unfortunately, there hasn't really been much effort to preserve this in the last 50 years, so the system is much more random today. But a few obvious clusters do still exist - 268/269/270 in SLC and 118/119/120 in Richfield being prime examples - as vestiges of a more organized system in the past.

Don't forget 7/8/9 all within ~30 minutes of each other around St. George.

I mean, there are others - you've got a bunch of 17x route numbers in the Salt Lake area, plus 81-83 all fairly close to each other in Box Elder (old SR 84 was part of that cluster as well).

The thing about the St George cluster that makes it unique is those route numbers are all newer. Just about all the other number clusters date back to when the numbers were first assigned.

[thspfc]

There are four 94s in the Milwaukee area; I-94, I-794, I-894, WI-794.
Three 41s in both the Appleton and Green Bay areas; I-41/US-41 plus WI-441 and US-141 respectively.
There have been five 41s total in the Milwaukee area, though only three exist now, and the most to exist at one time was four. US-41 and US-141 were the only ones there until 1980, when US-141 was truncated to Sheboygan at the south end. WI-241 and WI-341 were designated in 1999. I-41 was designated in April 2015. A bunch of routes were realigned in Milwaukee in August 2015 and WI-341 was decomissioned in the process. So today, I-41, US-41, and WI-241 are all the 41s in Milwaukee.

[NWI_Irish96]

Indiana has a grid system so this doesn't come up to much, but the Evansville area, including Posey, Gibson, Warrick and Vanderburgh Counties do feature several highways that are in the 60s. Hamilton County has four highways in the 30s but that's about it.

There are a few others. DeKalb County has 1, 3, 4 and 8
Wabash County has 13, 15, and 16, with 14 just to the north, 18 just to the south, and 19 just to the west.
Clinton County has 26, 28 and 29 with 22 and 25 not far away.
Monroe County has 43, 45, 46 and 48.
Greene County has 54, 57, 58 and 59.

[Bickendan]

Oregon follows a county based cluster for its Highways - Clatsop County has the 10x Highways, Multnomah the 12x (not counting OR 126), Washington 14x, etc, out to Malheur County's 45x highways. Because these are typically the secondary Highways, if they don't conflict with the same number Route, the Highway will be outright promoted to a Route for signage (OR 103, 104; the new OR 127).

West Bengal only has 15 numbered highways, 1-15, but they more or less tend to be in proximity to each other, with 12 and 12A being the odd one out in the northern handle of the state and no intersections with the other state highways. Several have long overlaps with NH highways, and every one is a cross-state highway in some regard.

[Flint1979]

In Michigan in Saginaw County there's M-46, M-47 and M-52 that are the closest together.

M-46 and M-47 use to run concurrent when M-47 ended further south being replaced by the northern half of M-52.

[bugo]

Arkansas.

Western Electric Model 500

[CapeCodder]

While not in the US, Newfoundland seems to group all of its route numbers together.

[CoreySamson]

TX doesn't really do that with their state primary routes, but there are some loose clusters of secondary examples.  I don't think the RMs have this happen nearly as much as their FMs.

A couple of examples:
Between Tyler and Rusk, there is FM 343, FM 344, FM 346, and FM 347.
West of Lake Palestine and mostly in Henderson County, there is FM 314, FM 315, FM 316, and FM 317.

I think this must be something with farm roads. Brazoria County has FM 521, FM 523, FM 524, and FM 528 within the county.

Brazoria County's county roads also have this problem. On SH 288 northbound between Angleton and Houston, you encounter CR 60, 61, 62, 63, 64, 56, 57, and 58 in that order (although why didn't they put 56, 57, and 58 before the others?) 

[Katavia]

Thanks to NCDOT's old ways of signing its state routes back in the day (for example, NC 160 would be the 10th branch of NC 16 - then NC 16 would be the 6th branch of NC 10), there are plenty of neighboring state routes close to each other... if you know where to look. The interstates and U.S. routes tore a lot of that system apart due to the new routes also having existing state routes with identical numbers, so they had to change them. Now they don't bother with keeping any scheme at all.

[SkyPesos]

In the Cincinnati metro area, there's OH 122, OH 123, OH 125, OH 126, US 127 (think this one is a coincidence), OH 128, OH 129, OH 130, OH 131, OH 132 and OH 133. OH 124 and OH 134 barely misses the metro area. From what it seems like, the preceding cluster of state routes are north of here at around Dayton, and the next cluster are east of the metro area.

[DTComposer]

The most noticeable cluster in California is probably 71, (relinquished) 72, 73, 74, 75, 76, 78, and 79 all appearing between L.A. and the Mexico border. 77, the exception, is used on a largely unconstructed route in Oakland.

A less noticeable example might be 236, 237, and 238 all being used for former sections of CA-9, with (all unconstructed) 230, 234, 235, and 239 all being in the vicinity. There is also 82, 84, 85, and 87 all interconnected in the Peninsula/South Bay.

[roadfro]

Aside from Illinois, I've also seen similar-numbered roads clustered together in Nevada, such as the many roads in the 580, 590, and 600 range in Vegas (in fact, I thought I read somewhere that Nevada actually assigns number ranges based on the county).

This is mostly correct. Nevada's 1976 renumbering reworked the federal aid highway numbers that the current state route numbers are derived from.

Federal aid secondary routes and state aid routes (100-400 series and 700-800 series state routes, respectively) were clustered by county alphabetically–so Carson City and Churchill & Clark Counties have numbers in the 100s and 700s while White Pine County has the high 400s and 800s. Of course there were the three numbering exceptions with adjacent state routes (28, 88, and 140), and there are routes that cross country lines where the number is valid for one county but not another (usually, the number was determined by whichever county contained the most mileage).

Federal aid urban routes were actually assigned by city alphabetically. This is why you see 500 & 501 in Boulder City (which may or may not have existed as actual state routes), 535 in Elko, 560s in Henderson, 570-610s in Las Vegas/North Las Vegas/unincorporated urban Clark County, and 640-680s in greater Reno-Sparks. Boulder City and Henderson are adjacent in Clark County but don't have adjacent numbers because of this.

[Bruce]

Washington's orderly numbering system results in clusters around major parent routes. In the Seattle area, half of the major routes are 5xx (in the 510-532 range) because they feed off I-5.

[OCGuy81]

Oregon follows a county based cluster for its Highways - Clatsop County has the 10x Highways, Multnomah the 12x (not counting OR 126), Washington 14x, etc, out to Malheur County's 45x highways. Because these are typically the secondary Highways, if they don't conflict with the same number Route, the Highway will be outright promoted to a Route for signage (OR 103, 104; the new OR 127).

West Bengal only has 15 numbered highways, 1-15, but they more or less tend to be in proximity to each other, with 12 and 12A being the odd one out in the northern handle of the state and no intersections with the other state highways. Several have long overlaps with NH highways, and every one is a cross-state highway in some regard.

Do the Oregon 2xx highways have an order to them? In the Willamette Valley it seems a lot of the state highways start with 2. 213, 214, 217, 219, 212, 224 to name a few

[TheStranger]

The most noticeable cluster in California is probably 71, (relinquished) 72, 73, 74, 75, 76, 78, and 79 all appearing between L.A. and the Mexico border. 77, the exception, is used on a largely unconstructed route in Oakland.

A less noticeable example might be 236, 237, and 238 all being used for former sections of CA-9, with (all unconstructed) 230, 234, 235, and 239 all being in the vicinity. There is also 82, 84, 85, and 87 all interconnected in the Peninsula/South Bay.

California in 1934 originally did a "cluster every four numbers" system, as seen here:

3 (now 1), 7 (now I-405), 11 (now 110), 15 (now 710), 19 in Los Angeles County
2, 6 (became a city street), 10 (later 42 and replaced by I-105 and I-5), 14 (now 91), 18, 22, 26, 30 between LA and Orange County
1, 5 (now 35), 9, 13 (renumbered to 17 and now 880/580) in the Bay Area
4, 8 (now 26), 12, 16, 20, 24 (remaining segment in Bay Area; this segment now 70) between Stockton and the northern Central Valley
23, 27 in the San Fernando/Simi Valley area
35 (replaced by 605), 39 in San Gabriel Valley
94 and 98 in the southernmost reaches of the state
134 and 138 in Los Angeles County
152 and 156 in Monterey County

1950s additions:
63 placed parallel to 65
76 near 78 in San Diego County
115 routed near 111 in Imperial County
154 supplanting a portion of what had been 150

Post-1964 clustering is a bit different, as seen by the examples you mentioned, and others here:

52, 54, 56 in metro San Diego
57 placed near 1934-present 55 in Orange County
(though 63 dates back to 1953)
72 and 73 placed near 74 in Orange County
82, 84, 85, 87 in the Bay Area
90 routed from and near (State Route/former US) 91
92 and 93 assigned to the Bay Area
112 and 114 assigned to the Bay Area
143 and 148 assigned to Sacramento as part of planned, unbuilt freeway system in the suburbs
original 163 and 165 and current unsigned 164 assigned to Los Angeles County
(former) 176 near 166 in Santa Maria
current 186 and 188 as border crossing spurs from I-8

(former) 212, 213, (former) 214, (original) 215 all in SoCal
former 224 and 225 near Santa Barbara
227 and 229 in San Luis Obispo County
(former) 231, 241, and 261 in Orange County
unbuilt 234 and 235 in Stockton
236, 237, 238 all as former segments of Route 9
(former) 248, unbuilt 249, and (former) 250 in SoCal
254 and 255 in far northwest California (both parallel loops to 101)
263, 265 and 273 as former routings of US 99 in far northern California
284 and (former) 285 near Route 70 in the Portola area