Busiest highways at state lines

[tolbs17]

I-77 just south of Charlotte to the SC border AADT is roughly 150,000.

I-85 between Grover NC and Blacksburg SC is roughly 40,000. Certainly not as busy but it's the second to the most busy

I-95 between the NC and VA state line, AADT is roughly 40,000

I-95 between MD and DE is roughly 65,000 AADT

I-495 going over the Potomac river is roughly 250,000 AADT

[NWI_Irish96]

I-80/94 at the IL/IN line is up close to 200k. Probably the biggest one in the Midwest.

[JayhawkCO]

GWB - 285,440.  The winner.

[TheHighwayMan3561]

MN's is I-94 across the St. Croix River (though the latest data for 2020 is probably dubious and I can't figure out how to access the more accurate previous year's numbers), though the Fargo/Moorhead crossing has grown rapidly and was 70K vehicles in 2019.

[Big John]

MN's is I-94 across the St. Croix River (though the latest data for 2020 is probably dubious and I can't figure out how to access the more accurate previous year's numbers)

The 2020 Wisconsin traffic count there in 72,200.  Busiest WI is the other I-94 border with IL at 76,300 (2020).

[Bruce]

I-205 at the Columbia River (Portland, OR/Vancouver, WA) - 166,000 AADT in 2019
I-5 at the Columbia River (Portland/Vancouver) - 138,500 AADT in 2019
I-82 at the Columbia River (Umatilla, OR/Plymouth, WA) - 21,700 AADT in 2016
I-90 at the Idaho state line - 56,000 AADT in 2020

[MATraveler128]

Surprised I-95 at the George Washington Bridge on the NY/NJ border hasn’t been mentioned yet. It’s one of the busiest bridges in the world.

[NWI_Irish96]

Surprised I-95 at the George Washington Bridge on the NY/NJ border hasn't been mentioned yet. It's one of the busiest bridges in the world.

It got mentioned 4 posts before yours.

[MATraveler128]

The Holland Tunnel in NYC.

[Flint1979]

The Brent Spence Bridge.
I-75 at the Michigan/Ohio line.

[SkyPesos]

Pretty much any major metro area straddling between states will have a busy highway at state lines. Some of them (NYC, Chicago, Cincinnati, DC, Portland) have been mentioned here already, and I think that Philadelphia, St Louis, Louisville, Memphis and Kansas City could also have some candidates here.

[ilpt4u]

Pretty much any major metro area straddling between states will have a busy highway at state lines. Some of them (NYC, Chicago, Cincinnati, DC, Portland) have been mentioned here already, and I think that Philadelphia, St Louis, Louisville, Memphis and Kansas City could also have some candidates here.

STL probably moreso before the Stan Span, as the Poplar Street Bridge carried Interstates 55, 64, 70, and arguably 44/Old US 66 traffic across the MS River

Part of the justification of the Stan Span was to spread some of the traffic around

[Scott5114]

I-435 at State Line Road in suburban Kansas City: 114,000

[webny99]

GWB - 285,440.  The winner.

Easily, followed by the Holland and Lincoln tunnels. Even I-287, Garden State Parkway, Palisades Parkway, and of course I-95 entering CT are pretty busy by non-NYC standards.

[jp the roadgeek]

I'm sure the Walt Whitman, Ben Franklin, and Delaware Memorial bridges all rank up there.

[KCRoadFan]

I'm sure that I-90 at the Wisconsin/Illinois border, near Beloit and Rockford, has to be right up there (especially in the summer), what with that being the main link between Chicago and Madison (and onward from there, the Dells and Minneapolis).

[Ted$8roadFan]

I'm sure I-95 at the state borders in all of the Northeast Corridor states from Washington, DC (Va/Md.) to the Boston area would have quite high volumes.

[Big John]

I'm sure that I-90 at the Wisconsin/Illinois border, near Beloit and Rockford, has to be right up there (especially in the summer), what with that being the main link between Chicago and Madison (and onward from there, the Dells and Minneapolis).

51,200 (2016 - date WI count was last done)

[tigerwings]

Michigan traffic counts per MDOT

I-75 at Ohio border - AADT (2020): 41,384
US-23 at Ohio border - AADT (2020): 43,253
I-69 at Indiana border - AADT (2020): 20,967
I-94 at Indiana border - AADT (2020): 44,883

[SkyPesos]

Related thread: https://www.aaroads.com/forum/index.php?topic=29110.25

I used AADT when listing Ohio border crossings there, and the state's 3 busiest are all between OH-KY in Cincinnati.

[SD Mapman]

I can tell you for SD it's definitely not SD 20 at the SD/MT border 

Actual AADT (2020) numbers (https://dot.sd.gov/media/documents/Traffic_2020.pdf):
I-29 at SD/IA: 35810
I-90 at SD/MN: 14000
US 81 at SD/NE: 8931
I-90 at SD/WY: 6920
SD 42 at SD/IA: 4669
I-29 at SD/ND: 4260
SD 20 at SD/MT: 92 (this has to be one of the smallest ones in the country lol)

[Mapmikey]

one crossing not super close to a city:

I-15 CA/NV was 45k pre-pandemic