Short, but Important

[nwi_navigator_1181]

There are a lot of routes that are short in nature, but are really important in the grand scheme of things. That tiny stretch makes the difference between the outskirts and downtown...or a quick jump on the interstate and a ride through surface streets. Which short streets serve a lot of importance? To keep it simple, here are my criteria:

The road must be ~5 miles TOTAL in length (not exact, within two or three miles either way is acceptable). So, please do not include "a portion of"  a highway as part of this (for example, the portion of I-65 between I-80/94 and the Indiana Toll Road, or the Edens Spur portion of I-94 doesn't count; they're still long routes in the grand scheme of things).

Unnumbered roads can qualify as long as they have a consistent amount of traffic; so sparsely traveled county roads do not count.

Examples:

Indiana 520 in Town of Pines: At less than a quarter mile in length, this small route plays a big role west of Michigan City, connecting US 12 and 20. Coming from the west, this could be the difference between downtown/lakeshore and access to the southern outskirts. Coming from Michigan City, it can either be a quick shot to Chesterton-Porter or access to the South Shore stations and the Steel Mills.

The same could be argued for Indiana 212 on Michigan City's east end. At three miles, it serves a similar purpose to Indiana 520 (right down to connecting US 12 and 20, along with US 35). Coming from Michigan, it can quickly put you deep into Indiana, while giving quick access to New Buffalo for those coming from the south. Serves as a decent alternative to I-94, which sits close by.

Indiana 249: the 2.5 mile stretch from US 12 to US 20 (sensing a pattern here) gives access to the lakefront, US Steel Midwest, the Port of Indiana, I-94, and the Ogden Dunes South Shore station, all within 10-15 minutes of each other). By extension, the Crisman Road-Willowcreek Bypass that starts immediately south of Indiana 249 provides further access to Downtown Portage and the Indiana Toll Road.

An interstate example: I-865 - which, let's face it, is one long ramp from I-65 south to the northwest corner of I-465 and the northwest corner of I-465 to I-65 north. As simplistic as that sounds, taking that spoke out of play would cause considerable trouble for those traveling within the Northwest Quadrant of Indianapolis. It can even be considered a gateway to Cincinnati and points southeast for those coming from Chicago. Close to five miles in length.

Any routes in your area that serve this purpose? Speak on it here. Thank you in advance for your responses.

[SkyPesos]

I-270 Spur MD. Much more tedious to get from I-70 to the NoVA suburbs of DC without it.

[NWI_Irish96]

There are several streets that connect 12 to 20 west of Michigan City so I'd argue that 520 isn't all that important, it just happens to have a number where the other streets don't.

249 is probably the most important of the group as it services Port of Indiana

212 is a vital route to bypass Michigan City going to/from New Buffalo and points north.

865 probably shouldn't even be its own route number.

IN 101/KY 1039 is the Markland Bridge over the Ohio and then connecting to I-71 so a vital route in that area.

[interstate73]

NJ/NY495 - 4.54 miles total from the Turnpike to 34th/Dyer, connecting the North Jersey suburbs, west-of-Hudson NY suburbs, Staten Island, and the rest of the country more broadly into Midtown Manhattan

[MATraveler128]

RI 37 in Cranston due to there being no direct connection from I-295 south to I-95 north and also serves TF Green Airport.

[Rothman]

I-238

[webny99]

The first one that came to mind was I-781. It's an important 4-mile connector between I-81 and Fort Drum, but arguably more importantly to the US 11 corridor and much of the North Country region, including Gouverneur, Canton, and Potsdam. Before I-781 was built, NY 342 served that movement, and it was one of the busiest two-lane roads in the state.

In the Syracuse area, NY 695 would fit. It's a short connector route between I-690 and NY 5 that provides an important connection in the freeway bypass of the Genesee St corridor.

And I know "sparsely traveled county routes" were explicitly excluded... but I still think Livingston CR 64 (Perry Rd) between NY 36 and US 20A might qualify. It's part of the fastest route between (a) the two largest cities in the state (Buffalo and NYC) and (b) the largest city in the US and the largest city in Canada (NYC and Toronto) which is pretty crazy when you think about it. The AADT is relatively low at 3800 (2019), but it would be more if not for the truck ban on US 20A through Warsaw.

[Rothman]

I-781 was only built because Fort Drum would have closed without it.  DOD demanded it.

[thspfc]

Airport spur freeways (I-190 IL, TX-97 Spur, WI-119, Pena Blvd for example)

[nwi_navigator_1181]

There are several streets that connect 12 to 20 west of Michigan City so I'd argue that 520 isn't all that important, it just happens to have a number where the other streets don't.

I agree with this 100%. There have been times where I connect using side streets. Mineral Springs Road, Indiana 49, Waverly Road, and Porter County Road 375 East (where the old Indiana Dunes Visitor Center used to be before it moved to its current location at the Indiana 49/US 20 interchange).

I mentioned 520 specifically because of the extreme close proximity of US 12 and 20 at that point (I believe this is the closest these two highways get without touching each other between MC and Gary, especially with the new split in place near Miller), as well as the fact this advantageous point is one of few points truckers are able to connect (the two other closest points being Indiana 49 and 212). From that standpoint, it at least deserves a nod.

[KCRoadFan]

Here in KC, I-670 comes to mind, being a more direct route for long-distance traffic going through downtown on I-70 compared to I-70 itself.

[Max Rockatansky]

CA 262 since the next directly accessing road between I-880 and I-680 to the north is I-238. 

[TheHighwayMan3561]

MN 280, which perhaps most prominently stood in for I-35W during the dark years of 2007-2009, but also serves as the main route to the Minnesota State Fairgrounds.

[kurumi]

For Connecticut, I looked for routes that:
* have no obvious parallel routes of same or greater class. So a high-traffic frontage road for I-95 doesn't count, for example
* has AADT of at least 25k (I have 2015 data, so it's likely a little higher)
* would create significant detour if taken out of service.

Here's the top contenders (freeways in bold):

SR 796: 2.7 miles, 28k

SR 722: 0.3 miles, 37k; connects 25/8 to US 1

SR 598: 0.55 miles, 30k; Conland-Whitehead Hwy, former I-484

SR 508: 1.07 miles, 37k; CT 4 connector to I-84

I-291, 6.05 miles, 64k

CT 229, 5.81 miles, 29k; I-84 to Bristol

CT 218, 6.7 miles, 38k; Hartford NW surface beltway

CT 73, 3.46 miles, 29k; spur from CT 8 to Watertown

If I had to pick a freeway, it would be SR 508. Traffic to Farmington and points west is already bad. Imagine if you had to use US 6 to Birdseye Road or something like that.

For a surface route: it's a close one, but I'll nominate CT 218. Imagine trying to cross southern Bloomfield E/W and the only options were 178 or 305.

[Techknow]

In California, I-980 was very very important when I-880 had to be reconstructed after the Loma Preita earthquake and is still very important, in Oakland, CA

https://www.aaroads.com/forum/index.php?topic=24601.msg2399295#msg2399295

[Dirt Roads]

Charleston, West Virginia has a couple notable ones: 

The unnumbered 35th Street/36th Street twin bridges over the Kanawha River serve as a connector from I-64/I-77 (eastbound only Exit 98) and ramps from US-60 beneath to cross the river over to Kanawha City.  That's about one mile, including streets on the Kanawha City side.

Also, the northernmost tip of Corridor G connects I-64/US-119 (Exit 58A) with the Southside Expressway (WV-61).  That section of Oakwood Road is about 1/4-mile in length, and it has a route number that is not posted: "US-119 Connector".

[elsmere241]

I-395 in Baltimore.

[7/8]

Some examples for Ontario, using TM lengths:
The 409 - 4.41 km freeway link between the 401, 427, and Pearson airport in Toronto
The 420 - 3.20 km freeway link from the QEW toward the Rainbow Bridge in Niagara Falls
Allen Road - 5.12 km freeway off the 401 in Toronto (I get the impression it's a congested mess, so not sure if it's really that useful )
Highbury Avenue - 4.55 km freeway from the 401 into London
The Linc (12.25 km freeway) and the Red Hill Valley Parkway (7.45 km freeway) make for a useful south bypass of Hamilton and serve the suburban "mountain" half of the city.
Highway 16 - 3.32 km road that connects the 416/401 in Johnstown to the Ogdensburg-Prescott Bridge (into NY).
Highway 85 - 9.64 km freeway off Highway 7 in Waterloo
Highway 148 - 7.34 km road that connects Pembroke to Quebec. Probably low traffic counts, but is one of the few road bridge crossings of the Ottawa River west of Ottawa.

[kphoger]

This might be an interesting math exercise:  AADT ÷ length

[hotdogPi]

This might be an interesting math exercise:  AADT ÷ length

About 5,000 AADT (personal estimate) divided by 350 feet: Fern St. in Methuen, MA

[Henry]

In Seattle, several freeways come to mind, but none more so than WA 520, WA 167, I-705 and WA 99's Alaskan Way.

[vdeane]

Before I-781 was built, NY 342 served that movement, and it was one of the busiest two-lane roads in the state.

From what I've heard, NY 342 was in fact the busiest two-lane road in the whole state before I-781 opened.

[hbelkins]

A few in my area:

KY 498. Known locally as "the cutoff road" and connects KY 52 to KY 11. A 2.5-mile route that bypasses what would otherwise be a 10-mile trip.

KY 1571. A ~five-mile route that cuts four miles of a steep, winding mountain crossing off KY 52 in Estill County.

KY 1425. This route originally had another name, but is now part of Man O'War Boulevard and it links I-75 to US 60 in Fayette County.

[JayhawkCO]

For Colorado, best I can come up with is I-270. It's 7.1 miles long, barely fitting in the OP's specs, but if it weren't there, the Mousetrap would be way more overrun that it already is. And, for the record, I despise I-270. It needs to be 2 lanes wider and have the buildup of trash left from the past few years cleaned up. But life in the metro would be worse without it.

[doorknob60]

I-405 in Portland, OR comes to mind. It's 4.25 miles long. I'd argue its most important use is connecting the US-26 freeway to I-5 (and by extension, I-84), though it also connects to US-30 and several downtown exits. I-405 is also one of only 2 freeway-grade bridges over the Willamette in Portland proper. Without I-405, the busy US-26 freeway, which carries a lot of through traffic due to no viable bypasses, would just dump into downtown streets.

I-238 was the first thing I thought of, but it was already mentioned.