Longest distance you can travel on just 3dis

[Flint1979]

In Michigan it'd just be the 81 miles of I-196, none of the 3di's in Michigan connect with each other so you'd could only do this using one Interstate.

You forgot that I-275 and I-696 meet northwest of Detroit. starting at I-94 and 11 Mile, you could take I-696 for 29 miles, then take I-275 south for 38 miles, the combination of I-696 and I-275 is almost 67 miles.

I know that I-275 and I-696 meet, but you still don't have the distance that I-196 is. Other than I-275 and I-696 none of Michigan's 3dis intersect with another 3di. In most states there would be more 3dis than what Michigan has since probably M-6, M-8, M-10, M-14, US-23, M-39, M-59, US-127 and US-131 would be Interstates.

[kirbykart]

Few more involving beltways:

- I-270 SB entire route, then loop around I-495 clockwise back to I-270 junction: about 100 miles

Make sure to head up I-295, then I-695, and I-395 back to the beltway for some extra mileage.

[dlsterner]

Theoretically if the 3di involved is a complete beltway, the answer is "infinity."

Nice try; unfortunately that is disallowed by the OP.

Rules:
1: No backtracking/looping back around to a previously traveled segment of the 3di.
2: You are allowed to cross state lines.
3: Concurrencies with 2dis are allowed as long as you only use the portion of the 2di that is actually concurrent with the 3di.
4: Let the fun begin!

[paulthemapguy]

SF Bay Area:  I-280 to I-880 to I-980 to I-580 to I-680, 165 or so miles.  As a bonus, you'll pass by I-380 and I-780 (and both ends of I-238).

I have a longer one in the Bay Area.  From I-80:  I-680, I-280, I-880, I-580.  About 175 miles.

There is no ramp from I-880 north to I-580 east-- but you could cut over on I-980 instead. It doesn't shave off much mileage.

[kirbykart]

I found a 68-mile one in Chicago area: I-355 north to I-290, east to I-294, take this to its southern terminus.
https://maps.app.goo.gl/rscGVBoMpczsPHmJ9

[ilpt4u]

84 miles is about the max that can be done in STL by only 3DIs, starting on I-270, just east of the I-170 interchange, heading EB, then exiting to I-255 SB in IL, circling the Beltway clockwise, continuing from 255 back to 270 back to the I-170 interchange, then take I-170 to its southern terminus at I-64

[TheHighwayMan3561]

MN gets 82 by driving a lap on I-494/694 with a start/end of the lap at I-394 and completion of 394 to/from its eastern terminus downtown.

[US 89]

I wonder how many states this is even worth calculating for. In the vast majority of western and plains states, there are no junctions between 3dis, so this answer is simply the length of the longest (or only) 3di. AZ and NM, of course, have none at all.

[1995hoo]

Few more involving beltways:

- I-270 SB entire route, then loop around I-495 clockwise back to I-270 junction: about 100 miles

Technically, starting with 270 SB, and then going clockwise around the Beltway, you can measure up to the 270/270 Spur split for the end point

I do wonder, tho, is this mileage greater than 270 SB, around 495 Counterclockwise, and then using (unsigned) 595 to its official endpoint in Annapolis, wherever exactly that is?

I tried this except that I exited I-495 onto I-395 > I-695 > I-295 back to I-495, then unsigned I-595. I-595 ends at MD-70. It's a total of 105 miles. If instead you stay on the Beltway back to I-270, you end up with 106 miles regardless of whether you go up I-270 to right before it merges back together with the I-270 Spur or whether you stay on the Beltway and count the mileage to right before the I-270 Spur merges into the Outer Loop.

[dlsterner]

Few more involving beltways:

- I-270 SB entire route, then loop around I-495 clockwise back to I-270 junction: about 100 miles

Technically, starting with 270 SB, and then going clockwise around the Beltway, you can measure up to the 270/270 Spur split for the end point

I do wonder, tho, is this mileage greater than 270 SB, around 495 Counterclockwise, and then using (unsigned) 595 to its official endpoint in Annapolis, wherever exactly that is?

I tried this except that I exited I-495 onto I-395 > I-695 > I-295 back to I-495, then unsigned I-595. I-595 ends at MD-70. It's a total of 105 miles. If instead you stay on the Beltway back to I-270, you end up with 106 miles regardless of whether you go up I-270 to right before it merges back together with the I-270 Spur or whether you stay on the Beltway and count the mileage to right before the I-270 Spur merges into the Outer Loop.

Getting a bit fictional, if I-97 had been numbered I-995 like some people here oddly want, you could get on I-995 from I-595 and travel up to I-695 and add in a tour of the Baltimore 3dis.  (But not gonna happen; I-97 forever!  )

[Dirt Roads]

This ones theoretical ATM.  But it should be doable in the future, ~240 miles: DE 495, DE/NJ/PA 295, PA 276, PA 476. 

But the PA turnpike interchange with I-295 isn't complete yet.

But you can do I-495(DE), I-295(DE/NJ) and I-195(NJ), which gives you a respectable 111.6 miles.

[Terry Shea]

In Michigan it'd just be the 81 miles of I-196, none of the 3di's in Michigan connect with each other so you'd could only do this using one Interstate.

You forgot that I-275 and I-696 meet northwest of Detroit. starting at I-94 and 11 Mile, you could take I-696 for 29 miles, then take I-275 south for 38 miles, the combination of I-696 and I-275 is almost 67 miles.

I know that I-275 and I-696 meet, but you still don't have the distance that I-196 is. Other than I-275 and I-696 none of Michigan's 3dis intersect with another 3di. In most states there would be more 3dis than what Michigan has since probably M-6, M-8, M-10, M-14, US-23, M-39, M-59, US-127 and US-131 would be Interstates.

You overlooked unsigned I-296 which does indeed intersect with I-196.  And since its length is listed as 3.43 miles and intersects I-196 at exit 77, the exact mileage of traveling all of I-296 along with the 77 or so miles of I-196 has to be very close to, if not surpassing the 80.630 miles officially listed as I-196.  It's probably a little short, but it has to be very, very close.

[1995hoo]

Few more involving beltways:

- I-270 SB entire route, then loop around I-495 clockwise back to I-270 junction: about 100 miles

Technically, starting with 270 SB, and then going clockwise around the Beltway, you can measure up to the 270/270 Spur split for the end point

I do wonder, tho, is this mileage greater than 270 SB, around 495 Counterclockwise, and then using (unsigned) 595 to its official endpoint in Annapolis, wherever exactly that is?

I tried this except that I exited I-495 onto I-395 > I-695 > I-295 back to I-495, then unsigned I-595. I-595 ends at MD-70. It's a total of 105 miles. If instead you stay on the Beltway back to I-270, you end up with 106 miles regardless of whether you go up I-270 to right before it merges back together with the I-270 Spur or whether you stay on the Beltway and count the mileage to right before the I-270 Spur merges into the Outer Loop.

Getting a bit fictional, if I-97 had been numbered I-995 like some people here oddly want, you could get on I-995 from I-595 and travel up to I-695 and add in a tour of the Baltimore 3dis.  (But not gonna happen; I-97 forever!  )

In that scenario, I came up with 180 miles.

[SkyPesos]

If you count state routes signed like 3di, there's a few options in the St Louis area involving I-170, I-270, I-255/IL 255, MO 364 or MO 370. Would be interesting to see which combination is the longest.

[KCRoadFan]

I wonder how many states this is even worth calculating for. In the vast majority of western and plains states, there are no junctions between 3dis, so this answer is simply the length of the longest (or only) 3di. AZ and NM, of course, have none at all.

Well, no one’s done Southern California yet…I’m sure there’s plenty!

Looking at the map, the longest one appears to involve going south on the 405 from its northern end on the 5 in Sylmar, then continuing south and east to pick up the 605 in Long Beach, taking that north to its end at the 210 in Duarte, and then heading west on the 210 back to the 5, basically coming full circle.

Based on the calculations I made from the exit lists on those roads’ Wikipedia pages, that drive would cover 111 miles. Can anyone find a longer continuous route among SoCal’s maze of 3di’s?

[ran4sh]

This ones theoretical ATM.  But it should be doable in the future, ~240 miles: DE 495, DE/NJ/PA 295, PA 276, PA 476. 

But the PA turnpike interchange with I-295 isn't complete yet.

But you can do I-495(DE), I-295(DE/NJ) and I-195(NJ), which gives you a respectable 111.6 miles.

Is I-495 considered to intersect with I-295 without needing to use I-95? Most maps I've seen show I-495/95 and I-295/95 as separate interchanges.

[Mapmikey]

This ones theoretical ATM.  But it should be doable in the future, ~240 miles: DE 495, DE/NJ/PA 295, PA 276, PA 476. 

But the PA turnpike interchange with I-295 isn't complete yet.

But you can do I-495(DE), I-295(DE/NJ) and I-195(NJ), which gives you a respectable 111.6 miles.

Is I-495 considered to intersect with I-295 without needing to use I-95? Most maps I've seen show I-495/95 and I-295/95 as separate interchanges.

There are direct ramps between 295 and 495 that do not require using any portion of 95.

[7/8]

Well, no one's done Southern California yet...I'm sure there's plenty!

Looking at the map, the longest one appears to involve going south on the 405 from its northern end on the 5 in Sylmar, then continuing south and east to pick up the 605 in Long Beach, taking that north to its end at the 210 in Duarte, and then heading west on the 210 back to the 5, basically coming full circle.

Based on the calculations I made from the exit lists on those roads' Wikipedia pages, that drive would cover 111 miles. Can anyone find a longer continuous route among SoCal's maze of 3di's?

I shared this one earlier:

About 124 miles in LA (using I-405, 110, 105, 710, 405, 605, and 210).

[Brandon]

I found a 68-mile one in Chicago area: I-355 north to I-290, east to I-294, take this to its southern terminus.
https://maps.app.goo.gl/rscGVBoMpczsPHmJ9

73 miles.  I-294 starts at I-94/80/IL-394.

[kirbykart]

I wonder how many states this is even worth calculating for. In the vast majority of western and plains states, there are no junctions between 3dis, so this answer is simply the length of the longest (or only) 3di. AZ and NM, of course, have none at all.

Well, no one's done Southern California yet...I'm sure there's plenty!

Looking at the map, the longest one appears to involve going south on the 405 from its northern end on the 5 in Sylmar, then continuing south and east to pick up the 605 in Long Beach, taking that north to its end at the 210 in Duarte, and then heading west on the 210 back to the 5, basically coming full circle.

Based on the calculations I made from the exit lists on those roads' Wikipedia pages, that drive would cover 111 miles. Can anyone find a longer continuous route among SoCal's maze of 3di's?

7/8 found a 124-mile route in the LA area.

[kirbykart]

I found a 68-mile one in Chicago area: I-355 north to I-290, east to I-294, take this to its southern terminus.
https://maps.app.goo.gl/rscGVBoMpczsPHmJ9

73 miles.  I-294 starts at I-94/80/IL-394.

I saw that on Google Maps, but I wasn't sure if that was correct because sometimes Google Maps does weird things. If you couldn't tell, I don't know very much about Chicago.

[Flint1979]

I found a 68-mile one in Chicago area: I-355 north to I-290, east to I-294, take this to its southern terminus.
https://maps.app.goo.gl/rscGVBoMpczsPHmJ9

73 miles.  I-294 starts at I-94/80/IL-394.

I saw that on Google Maps, but I wasn't sure if that was correct because sometimes Google Maps does weird things. If you couldn't tell, I don't know very much about Chicago.

This is the part you were missing, https://www.google.com/maps/dir/41.5780035,-87.6672829/41.5776427,-87.5817892/@41.5771732,-87.6317661,13z/data=!4m2!4m1!3e0

[bing101]

I wonder how many states this is even worth calculating for. In the vast majority of western and plains states, there are no junctions between 3dis, so this answer is simply the length of the longest (or only) 3di. AZ and NM, of course, have none at all.

Well, no one's done Southern California yet...I'm sure there's plenty!

Looking at the map, the longest one appears to involve going south on the 405 from its northern end on the 5 in Sylmar, then continuing south and east to pick up the 605 in Long Beach, taking that north to its end at the 210 in Duarte, and then heading west on the 210 back to the 5, basically coming full circle.

Based on the calculations I made from the exit lists on those roads' Wikipedia pages, that drive would cover 111 miles. Can anyone find a longer continuous route among SoCal's maze of 3di's?

How about this one I-405 south to I-105 east, I-605 North to I/CA-210 east to I-215? If that works or not.
Have another one too CA/I-110 south , to I-405 South, I-710 North, I-105 east, I-605 North, I/CA-210 east to I-215 North or South.

Some of this is that 110 and 210 lose their 3di status in some parts of the route?

[kirbykart]

I wonder how many states this is even worth calculating for. In the vast majority of western and plains states, there are no junctions between 3dis, so this answer is simply the length of the longest (or only) 3di. AZ and NM, of course, have none at all.

Well, no one's done Southern California yet...I'm sure there's plenty!

Looking at the map, the longest one appears to involve going south on the 405 from its northern end on the 5 in Sylmar, then continuing south and east to pick up the 605 in Long Beach, taking that north to its end at the 210 in Duarte, and then heading west on the 210 back to the 5, basically coming full circle.

Based on the calculations I made from the exit lists on those roads' Wikipedia pages, that drive would cover 111 miles. Can anyone find a longer continuous route among SoCal's maze of 3di's?

How about this one I-405 south to I-105 east, I-605 North to I/CA-210 east to I-215? If that works or not.
Have another one too CA/I-110 south , to I-405 South, I-710 North, I-105 east, I-605 North, I/CA-210 east to I-215 North or South.

Some of this is that 110 and 210 lose their 3di status in some parts of the route?

I won't allow this because as you said, 110 and 210 lose their 3di status, and this thread is about traveling on just 3dis. 7/8's route stays on the 3di portions of these roads, so that's OK.

[ilpt4u]

I found a 68-mile one in Chicago area: I-355 north to I-290, east to I-294, take this to its southern terminus.
https://maps.app.goo.gl/rscGVBoMpczsPHmJ9

73 miles.  I-294 starts at I-94/80/IL-394.

Once I-490 is built and opens and should IL 390 get promoted to I-390, then Chicago would get a little longer