The Furthest Into A State Can You Go On An Interstate....

[Eth]

For Georgia, it's I-59 northbound from Alabama. Exit 4 is for a county road; the next interchange is Exit 11 in Trenton for GA 136. None of the other crossings (I-20, I-24, I-75, I-85, I-95, I-520) make it any more than about five miles.

[Urban Prairie Schooner]

Louisiana (distances are approximate):

I-10 west entrance: 4 miles (US 90/LA 109)
I-10 east entrance: 6 miles (I-12/59)
I-20 west entrance: 3 miles (LA 169)
I-20 east entrance: 3 miles (US 80)
I-55 entrance: 4 miles (LA 38)
I-59 entrance: 6 miles (LA 3081 [formerly LA 41 Spur])

[MDOTFanFB]

After I-75 from the south (already mentioned), Michigan's longest is on I-69, with 13 miles between the Indiana state line and U.S. 12 at exit 13. All other unmentioned Michigan interstate entrances only intersect a signed route within a mile of the state line, except I-75 from the north, which doesn't intersect a signed route until three miles later.

[iowahighways]

Interesting related question: what's the farthest between exits for state highways? I think Florida's Turnpike is still the longest between exits, period (49 miles if you get on northbound at Yeehaw Junction), but there have to be longer state highway examples out west.

For Iowa, the longest gap is on I-35: 25 miles between IA 3 and US 18/IA 27. The longest gap on I-80 is 24 miles between IA 25 and US 169, the longest gap on I-29 is 23 miles between IA 127 and IA 175, and the longest gap on I-380 is 22 miles between IA 150 and US 20.

[Duke87]

I-15 in the same state has 44 miles before MT 324.

Close, except exit 0 is a state highway!

I dunno, I took the OP's use of the term "shielded route" to imply that unsigned highways don't count. If we go by maintenance rather than by signage this changes a lot of answers and makes them more difficult to determine since you can't just peruse an ordinary map. I would never have guessed that that dinky road was state maintained!

This also then begs the question: in West Virginia, where nearly 90% of all public road mileage is state maintained, is there an exit off an interstate which leads to one which is not?

[Alps]

NY: I-81 across the Thousand Islands appears to be the sad winner, to be beaten out by I-99.

[vdeane]

Actually, the Thruway goes 11 miles from the PA border before hitting NY 394 in Westfield.  The Thruway Authority built the Ripley exit at a random local road rather than NY 76 so it could be close to the PA border (which looks idiotic, except Ripley probably wouldn't have gotten an exit at all if it weren't on top of the border).

EDIT: And for the second question, the longest in NY is possibly the Thruway between NY 394 and NY 60 in Dunkirk, at 17 miles, but I've only given this two seconds of thought. NY 17 may beat the Thruway here NY 17 between NY 19 and NY 21 (19 miles).

[jp the roadgeek]

I-684 goes through CT without encountering a single state route or US route

Actually, the pathetic winner for CT has to be I-395 from the MA line to the CT 200 exit, which is maybe 3-4 miles tops.  RI winner has to be I-295 from MA (about 3.7 miles).

[Takumi]

On I-95 in Virginia, you can go nearly 31 miles from the NC border without crossing a primary route.

US 58, at 11 miles, and US 301, at 17 miles, are primary routes.

And exit 8 is for US 301. They don't actually cross, but the OP asks how long you can go before an exit for a state or US route. So I-95 northbound is 8 miles. I-95 southbound is about a mile, I-64 eastbound is 10 miles (US 60 departure and VA 159; westbound doesn't count), I-66 westbound about the length of this post (US 50; eastbound doesn't count), I-81 southbound 7 miles (US 11), I-81 northbound 1 mile (US 58/421), I-85 northbound 12 miles for the win (US 58; southbound doesn't count).

[Beeper1]

In CT, the longest into the state would be I-395 (4 miles from MA border to CT-200). 

In MA, since Exit 1 on the Mass Pike is westbound only, that makes it 10 miles from the NY line to the first state highway that traffic coming into the state can exit at in Lee at US-20.

In RI, the longest would be just 1 mile, on I-295 from the MA line to RI-114.

Vermont would be a tie at 7.5 miles on I-91 from MA line to US-5 in Brattleboro and on I-93 from the NH line to VT-18.

Both Maine and NH would be less than a mile, as all entering interstates have interchanges almost right at the state lines.

[NE2]

In RI, the longest would be just 1 mile, on I-295 from the MA line to RI-114.

Actually 3 miles on I-195, due to partial interchanges and the fact that the only westbound exit in East Providence is onto a local street (the guide signs are missing 'TO', but the assembly on the ramp correctly includes it). Unless you believe RI 152 still goes down that way.

[Alps]

Actually, the Thruway goes 11 miles from the PA border before hitting NY 394 in Westfield.  The Thruway Authority built the Ripley exit at a random local road rather than NY 76 so it could be close to the PA border (which looks idiotic, except Ripley probably wouldn't have gotten an exit at all if it weren't on top of the border).

But that exit is mere feet from a state highway. How precise are we being, here?

[NE2]

Isn't that "random local road" NY 950D?

[getemngo]

After I-75 from the south (already mentioned), Michigan's longest is on I-69, with 13 miles between the Indiana exit and U.S. 12 at exit 13. All other unmentioned Michigan interstate entrances only intersect a signed route within a mile of the state line.

You're forgetting I-75's northern end. 3 whole miles from the marked border on the International Bridge to BS I-75 at Exit 392! 

(And for those not familiar with the area... no, that's not the first exit, either.)

[Kacie Jane]

For Washington, it's I-90, about 10 miles coming in from Idaho before you hit a state route.  On I-5, both on the north and south ends the first exit is a state route, and both within half a mile.  Ditto for I-205 coming in from Oregon, but it's closer to a full mile from the state line in the middle of the river before you hit SR 14.

Edit:  Crap, I forgot I-82.  However, there too the first exit is for SR 14, 1.02 miles from the state line.

[ysuindy]

*Fun fact*
The first (or last, depending on your direction of travel) interchange along I-80, I-70, I-470, and I-77 in eastern Ohio is with State Route 7 (7A, in the case of I-470).

Isn't the first / last exit on I-90 also State Route 7?

[mgk920]

*Fun fact*
The first (or last, depending on your direction of travel) interchange along I-80, I-70, I-470, and I-77 in eastern Ohio is with State Route 7 (7A, in the case of I-470).

Isn't the first / last exit on I-90 also State Route 7?

Well, not as many, but the first state highway met from the west in Wisconsin by I-90, I-94 and I-535 (as well as by US 61/151 from the southwest and US 2, US 8 and US 18 from the west) is WI 35.

Mike

[mapman1071]

Arizona

From Colorado River Midpoint (California Line)
I-8 ,I-10 & I-40 Have  A Exit 1 at or before MM 1

From Colorado River Midpoint (Nevada Line)
I-15 First Exit Is 8 (MM 8)

From Business I-19 Intersection. Nogales
I-19 Exit 1A Southbound (KM .71 710M) Exit 1 NB/Exit 1B Sb (KM 1.9 1900m)


I-10 Last Exit Is 390 (MM 390) At The New Mexico Line
I-40 Last Exit Is 359 (MM 359) At The New Mexico Line
I-15 Last Exit is 27 (MM 27) Utah Line is at MM 29

[Bickendan]

Oregon: [California] I-5 to OR 273 - 1 mile
[Washington] I-5 to OR 99E - 1 mile
[Washington] I-82 to US 395/730 - 1 mile
[Idaho] I-84 to US 30 - 1 mile

Idaho: [Utah] I-15 to ID 38 - 13 miles
[Montana] I-15 to ID 22 - 29 miles
[Oregon] I-84 to US 95 - 3 miles
[Utah] I-84 to I-86/US 30 - 53 miles
[Washington] I-90 to US 95 - 12 miles
[Montana] I-90 to ID 4 - 12 miles

[JustDrive]

CALIFORNIA:

I-5 from Oregon:  SR 96 (11 miles)
I-80 from Nevada:  SR 89/267 (17 miles)
I-15 from Nevada:  SR 127 (50 miles)
I-40 from Arizona:  US 95 (11 miles)
I-10 from Arizona:  enters co-signed with I-10; both routes split 3 miles west of the AZ/CA state line.
I-8 from Arizona:  SR 186 (6 miles)

US Routes (because there are so few in California):

US 101 from Oregon: SR 197 (10 miles)
US 199 from Oregon: SR 197 (32 miles)
US 97 from Oregon:  SR 161 (.28 miles)
US 395 from Oregon: SR 299 (33 miles)
US 395 from Nevada (north end): SR 70 (8 miles)
US 50 from Nevada: SR 89 (5 miles)
US 395 from Nevada (south end): SR 89 (5 miles)
US 6 from Nevada: SR 120 (7 miles)
US 95 from Nevada:  I-40 (23 miles)
US 95 from Arizona:  enters co-signed with I-10

[sandwalk]

*Fun fact*
The first (or last, depending on your direction of travel) interchange along I-80, I-70, I-470, and I-77 in eastern Ohio is with State Route 7 (7A, in the case of I-470).

Isn't the first / last exit on I-90 also State Route 7?

Oh yeah....missed that one! Thanks!

[WNYroadgeek]

Actually, the Thruway goes 11 miles from the PA border before hitting NY 394 in Westfield.  The Thruway Authority built the Ripley exit at a random local road rather than NY 76 so it could be close to the PA border (which looks idiotic, except Ripley probably wouldn't have gotten an exit at all if it weren't on top of the border).

But that exit is mere feet from a state highway. How precise are we being, here?

I would assume it counts because the interchange itself doesn't intersect a state highway, despite being close to one.

[NE2]

Actually, the Thruway goes 11 miles from the PA border before hitting NY 394 in Westfield.  The Thruway Authority built the Ripley exit at a random local road rather than NY 76 so it could be close to the PA border (which looks idiotic, except Ripley probably wouldn't have gotten an exit at all if it weren't on top of the border).

But that exit is mere feet from a state highway. How precise are we being, here?

I would assume it counts because the interchange itself doesn't intersect a state highway, despite being close to one.

Isn't that "random local road" NY 950D?

[vdeane]

This brings up the question as to whether reference routes count.

[froggie]

Well, not as many, but the first state highway met from the west in Wisconsin by I-90, I-94 and I-535 (as well as by US 61/151 from the southwest and US 2, US 8 and US 18 from the west) is WI 35.

Did WisDOT decommission WI 87? :o)

Also, I'm not 100% on the signage status of the new truck bypass in Prairie du Chien, but if US 18 is fully on the bypass, its first route would now likely be WI 27.  Or TRUCK US 18.