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View Full Version : What's the biggest steepest (interstate) hill in the whole USA?


Squink
09-04-2005, 09:35 PM
This thread (http://boards.straightdope.com/sdmb/showthread.php?p=6544066#post6544066) got me wondering about hills on the intersate.
There's that long steep grade coming into Salt Lake City on 80 from the east, 10 or 15? into LA, and some of the mountain crossings on 90 in Pennsylvania are no slouches either.
What's the longest grade on the interstate system. What's the steepest?

Uvula Donor
09-04-2005, 10:47 PM
The steepest grade on the Interstate system is on I-24/US-64 Westbound over Monteagle Mountain, TN. It's an 8% grade. Cite. (http://www.roadfan.com/mtrfaq.html#a69)

Don't know about the longest, though.

Cunctator
09-04-2005, 11:52 PM
You may find some details in this thread. (http://boards.straightdope.com/sdmb/showthread.php?t=321124&highlight=baldwin+dunedin)

snailboy
09-05-2005, 12:16 AM
The steepest grade on the Interstate system is on I-24/US-64 Westbound over Monteagle Mountain, TN. It's an 8% grade. Cite. (http://www.roadfan.com/mtrfaq.html#a69)

That can't be right. I-70 gets steeper than that crossing the Rocky Mountains. I believe it was a 15% grade where I put my car in neutral and let it coast up to 115 MPH before finally hitting the brakes.

The Mad Hermit
09-05-2005, 07:02 AM
There's that long steep grade coming into Salt Lake City on 80 from the east, 10 or 15? into LA, and some of the mountain crossings on 90 in Pennsylvania are no slouches either.

I-90? It runs through one of the flattest parts of the state, the Lake Erie shore.

You may mean I-80. The Turnpike, I-76, had tunnels cut to avoid the steeper slopes. US 30 from Ligonier to Chambersburg is a good measure of the highest PA road, but it's not an interstate.

Athena
09-05-2005, 07:18 AM
I don't know about longest or steepest, but I-70 going either way through the Eisenhower tunnel (the highest vehicular tunnel in the world, at slightly higher than 11K feet) you get a long, steep downhill. Can be very scary when the traffic is already going at 80 mph.

Big Whistle
09-05-2005, 08:17 AM
That can't be right. I-70 gets steeper than that crossing the Rocky Mountains. I believe it was a 15% grade where I put my car in neutral and let it coast up to 115 MPH before finally hitting the brakes.
There is no way there is a 15% grade on any interstate highway. Certainly not on the section of I-70 either east or west of the tunnel, or at any of the other steep grades, such as out of Vail.

Uvula Donor
09-05-2005, 08:19 AM
That can't be right. I-70 gets steeper than that crossing the Rocky Mountains. I believe it was a 15% grade where I put my car in neutral and let it coast up to 115 MPH before finally hitting the brakes.

I believe you are wrong. I've provided a cite which includes the standard approved for the Interstate highway system, noting that a waiver had to be granted for the 8% grade on that stretch. Provide a valid cite for your 15%.

Uvula Donor
09-05-2005, 08:21 AM
You may find some details in this thread. (http://boards.straightdope.com/sdmb/showthread.php?t=321124&highlight=baldwin+dunedin)

Not likely; the OP is asking about the American Interstate highway system, not individual city streets.

St. Urho
09-05-2005, 08:27 AM
When I lived in Arizona, people said that there was a climb on I-17 northbound out of Phoenix that was the steepest interstate grade in the U.S. I'll see if I can get in touch with someone for a cite.

St. Urho
09-05-2005, 08:35 AM
Annnd it looks like I-17 just gets to 7%, but at 2 or 3 places.

Cisco
09-05-2005, 12:06 PM
I-17 northbound out of Phoenix
It's reeeally curvy, too. I didn't really notice it last weekend when we were northbound, but coming back was a different story. It was dark, I was worn out from climbing Humphreys Peak that day, traffic was thick but moving full-speed, including lots of tractor-trailer trucks, and the speed limit through that stretch is an ungodly 75mph. After sweating through it for a few miles I finally looked over to my wife and said, "I don't want to sound like a wuss, but for the first time in my life I don't feel comfortable going the speed limit," and slowed down to about 65ish. Got to be one of the most dangerous stretches of interstate, especially considering there can't be any hospitals too awful close.

Squink
09-05-2005, 12:15 PM
I-90? It runs through one of the flattest parts of the state

You may mean I-80.Probably. It's been ages since I've driven through Penn to NY.

Kimstu
09-05-2005, 12:23 PM
It was dark, I was worn out from climbing Humphreys Peak that day, traffic was thick but moving full-speed, including lots of tractor-trailer trucks, and the speed limit through that stretch is an ungodly 75mph. After sweating through it for a few miles I finally looked over to my wife and said, "I don't want to sound like a wuss, but for the first time in my life I don't feel comfortable going the speed limit," and slowed down to about 65ish.



What's "wussy" about driving under the speed limit? The speed limit is supposed to be an approximate upper bound on how fast it's generally safe to go on a particular stretch of road under good conditions. You're supposed to drive more slowly on average under bad conditions, including darkness, fatigue, unfamiliarity with the road, heavy traffic, and a steep grade---or in your case, all of the above.

I don't mind being on the road with drivers going the speed limit (or even a reasonable amount above it) because they're comfortable with that speed under those conditions. But Lawsy, I don't want them driving fast, against their better judgement, just because they don't want to seem like a "wuss". Thank you for driving safely.

Jake
09-05-2005, 06:48 PM
Try driving an 18 wheeler from Reno to Sacramento. :eek: Downhill for I don't remember how many miles. A bunch.
The grade outside Salt Lake going east is a real bitch.
I think the worst I've pulled is over the "Grapevine" from Bakersfield to LA. Either direction. On a hot day.

enipla
09-06-2005, 07:57 AM
That can't be right. I-70 gets steeper than that crossing the Rocky Mountains. I believe it was a 15% grade where I put my car in neutral and let it coast up to 115 MPH before finally hitting the brakes.As others have said, Eisenhower tunnel approaches are 7-8%. I work for the county that half the tunnel is located in. My driveway is 15-17%. No comparison.
From this site -
http://www.dot.state.co.us/eisenhower/description.asp

The approach grades are steep, being 7 percent on the west approach and 6 percent on the east approach.

Absolute
09-06-2005, 08:25 AM
What's "wussy" about driving under the speed limit? The speed limit is supposed to be an approximate upper bound on how fast it's generally safe to go on a particular stretch of road under good conditions. You're supposed to drive more slowly on average under bad conditions, including darkness, fatigue, unfamiliarity with the road, heavy traffic, and a steep grade---or in your case, all of the above.

I don't mind being on the road with drivers going the speed limit (or even a reasonable amount above it) because they're comfortable with that speed under those conditions. But Lawsy, I don't want them driving fast, against their better judgement, just because they don't want to seem like a "wuss". Thank you for driving safely.

It's been shown that drivers will drive at the speed they feel comfortable with, regardless of the speed limit.

In most places in the US, the speed limit is set ridiculously low compared to the speed of traffic. Why this is, I don't know. But at least here in the northeast, it is usually set at about 15 mph slower than the flow of traffic. Thus, the majority of the people on the road feel comfortable doing 15 over the limit. Which is not unsafe by any means. It means going 70 or 80 on highways. But the claim that it is an "approximate upper bound" on speeds in any weather/traffic conditions is ludicrous. Hell, I've had state police tell me that anything under 80 is fine, in a 55 mph zone!

This means that there is something wrong with the few people who insist on going the speed limit, or below it. Mechanical problems aside, the usual explanation is that they're a "wuss": either terrified of breaking the law, or terrified of driving fast.

In any case, these people are a danger to other traffic. If the speed limit on a road is 55 mph, and Bob Slowpoke thinks that 50 mph is a safe and reasonable speed, he's putting himself and other cars at risk by driving 20 mph slower than the rest of traffic.

When I drive into other states outside New England, I'm amazed that the speed limits roughly correspond to the speed of traffic.

don't ask
09-06-2005, 08:43 AM
In most places in the US, the speed limit is set ridiculously low compared to the speed of traffic. Why this is, I don't know.

I don't know about the US but in Australia it is an historic artifact. When I was studying Land and Engineering Surveying we had to do road designs over all sorts of terrain. The tricks you had to learn were how to avoid step inclines, how to avoid cuts and fills that required shipping away soil/rock and, most importantly, how to engineer the corners so that the passenger was comfortable in an average car driven by a driver of average ability.

If I did my calculations correctly 30 odd years ago I may have set a bend at 80kph (50mph) but in my modern car with low profile tyres I can zip around at 100kph with no problems. However to adjust the signs to allow for modern cars would entail replacing every one overnight, otherwise some would be to one standard and others would not. And we are talking many many thousands of signs...so they recommend speeds for 30 year old cars.

Kimstu
09-06-2005, 09:33 AM
Well, this is continuing a hijack, but just to clear up some misunderstandings:


But at least here in the northeast, it [the speed limit] is usually set at about 15 mph slower than the flow of traffic. Thus, the majority of the people on the road feel comfortable doing 15 over the limit. Which is not unsafe by any means. It means going 70 or 80 on highways. But the claim that it is an "approximate upper bound" on speeds in any weather/traffic conditions is ludicrous.


Then you must be defining "approximate" to mean "within about 5 mph", which I think is too restrictive, and not what I meant. I don't have any problem with people driving, say, 15 mph over the speed limit, or even faster, if they can do so safely and comfortably. If they're willing to risk a speeding ticket, that's their business.


This means that there is something wrong with the few people who insist on going the speed limit, or below it. Mechanical problems aside, the usual explanation is that they're a "wuss": either terrified of breaking the law, or terrified of driving fast.

In any case, these people are a danger to other traffic. If the speed limit on a road is 55 mph, and Bob Slowpoke thinks that 50 mph is a safe and reasonable speed, he's putting himself and other cars at risk by driving 20 mph slower than the rest of traffic.


I don't buy it. Multi-lane highways need to be able to accomodate a fairly wide range of speeds, and people who drive on them should be prepared for that. Demanding that all highway traffic fall within a speed range of less than 20mph variation between the slowest and fastest drivers is unrealistic and unfair. Highways are there to serve drivers with a wide variety of vehicle types, driving skills, and speed preferences. (Not an unlimited variety, mind you---people should not try to drive, say, 25mph in freely-moving interstate traffic---but a reasonably wide variety that accomodates minimum speeds at least 5--10mph under the speed limit.)

If you don't like that, then campaign for special "autobahn" highways with high minimum speeds, so the slower drivers have to stay off them. But on an ordinary highway, if you can't drive safely just because another driver is going 5mph under the speed limit, then you are the one who's putting traffic at risk, not the slower driver.

Now, if you were making this argument specifically about the left lane, I would heartily agree with you. Slower drivers are entitled to be on highways, but they need to keep to the right. (And also, as don't ask noted, it's important to keep posted speed limits up-to-date to reflect changes over time in road conditions and vehicle capabilities.)

In any case, it is still monumentally stupid to drive faster than you can safely manage merely from fear of seeming like a "wuss". If you're driving too fast to maintain proper control of your vehicle, that's far more dangerous to you and to others than if you're driving 5mph under the speed limit.

Kimstu
09-06-2005, 09:36 AM
Oh, and by the way:


It's been shown that drivers will drive at the speed they feel comfortable with, regardless of the speed limit.

In most places in the US, the speed limit is set ridiculously low compared to the speed of traffic. [...]

When I drive into other states outside New England, I'm amazed that the speed limits roughly correspond to the speed of traffic.


Make up your mind: are speed limits in most of the US, outside New England, "ridiculously low compared to the speed of traffic" or do they "roughly correspond to the speed of traffic"?

Absolute
09-06-2005, 10:07 AM
Oh, and by the way:



Make up your mind: are speed limits in most of the US, outside New England, "ridiculously low compared to the speed of traffic" or do they "roughly correspond to the speed of traffic"?
You're right, I changed my mind halfway through and realized that I've only noticed this in the northeast.

snailboy
09-06-2005, 12:26 PM
As others have said, Eisenhower tunnel approaches are 7-8%. I work for the county that half the tunnel is located in. My driveway is 15-17%. No comparison.
From this site -
http://www.dot.state.co.us/eisenhower/description.asp

Well that's surprising. I thought it was in the double digits, but I guess my memory failed me. It certainly seems steeper, being able to coast so fast.

Kevbo
09-06-2005, 01:29 PM
Well that's surprising. I thought it was in the double digits, but I guess my memory failed me. It certainly seems steeper, being able to coast so fast.

On a hot summer day (say 70F ) the air density is only about 57% the sea level value, thus aerodynamic drag is reduced by over 40% for a given speed. But since drag increases as the square of speed, the net result is that you'd coast around 1/3 faster at 11,000' MSL and 70F than at sea level and 65F.

So if you coasted to 115mph up there, you'd only make 86 at sea level.

Kevbo
09-06-2005, 01:32 PM
Oh yes, the power of a normally aspirated, fuel injected engine is down by that 43% as well, so the uphill bits seem all the more steeper.

Carburated engines do far worse, as the mixture goes way rich.

snailboy
09-06-2005, 03:38 PM
On a hot summer day (say 70F ) the air density is only about 57% the sea level value, thus aerodynamic drag is reduced by over 40% for a given speed. But since drag increases as the square of speed, the net result is that you'd coast around 1/3 faster at 11,000' MSL and 70F than at sea level and 65F.

So if you coasted to 115mph up there, you'd only make 86 at sea level.

Interesting. I hadn't thought about the decreased drag. My car is already good on aerodynamics. I was still accelerating pretty well at 115, but after I hit my speed limiter at 109, the engine died and I didn't enjoy flying down a downhill curve without power brakes or power steering. So anyone who's interested in seeing what happens when you exceed your speed limiter in neutral should just read my post and forget about trying it. Alright, I'll quit going off on a tangent in this thread now.

Mr. Slant
09-06-2005, 04:02 PM
SNIP

If I did my calculations correctly 30 odd years ago I may have set a bend at 80kph (50mph) but in my modern car with low profile tyres I can zip around at 100kph with no problems. However to adjust the signs to allow for modern cars would entail replacing every one overnight, otherwise some would be to one standard and others would not. And we are talking many many thousands of signs...so they recommend speeds for 30 year old cars.

Is there any place online I could find these formulas?
I'd be really curious to work them myself and see how various roads with known traits work out.

Necros
09-06-2005, 04:19 PM
Interesting. I hadn't thought about the decreased drag. My car is already good on aerodynamics. I was still accelerating pretty well at 115, but after I hit my speed limiter at 109, the engine died and I didn't enjoy flying down a downhill curve without power brakes or power steering. So anyone who's interested in seeing what happens when you exceed your speed limiter in neutral should just read my post and forget about trying it. Alright, I'll quit going off on a tangent in this thread now.

Your engine SHUT OFF when you hit the speed limited in your car?!? That is unbelieveably dangerous, especially on a road like 70 goming down from Eisenhower. What kind of POS are you driving?

Glad I don't have to worry about it. My car's top speed is limited by drag. Right now, about 180 mph. ;)

snailboy
09-06-2005, 08:40 PM
Your engine SHUT OFF when you hit the speed limited in your car?!? That is unbelieveably dangerous, especially on a road like 70 goming down from Eisenhower. What kind of POS are you driving?

It's a 94 Thunderbird LX. The speed limiter works by completely cutting the fuel injectors. Apparently Ford didn't program the EEC to make sure the car was in gear before doing it. To be honest, I doubt many car's computers do. How often do people roll down the Rocky Mountains in neutral past the speed limiter? Maybe I should write them to let them know about this glitch.

t-bonham@scc.net
09-07-2005, 01:55 AM
I can't find anything about what the actual grade is, but I-35 coming over the hill and descending into Duluth, MN is one I remember as pretty steep (and twisty, too). And a favorite spot for Highway Patrol to hide, to try to make their quota of speeding tickets from people suddenly on this steep downhill road.

It's probably not as steep as the old Highway-61* was, though. Anybody know just what grade it is?


* Yes, this is the Highway-61 of the Bob Dylan song.

zagloba
09-07-2005, 02:46 AM
True, but Dylan is referring to the southern end, where it's the Blues Highway, perhaps more than the northern end, where it passes near his birthplace.