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Old March 13th, 2010, 10:40 AM   #5441
H123Laci
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Originally Posted by pwalker View Post
I didn't see any real bad driving in this video. For the most part, everyone seemed courteous and alert. Add in left-exits, which are more common than most would believe, and this is quite normal.

left-exit is unsafe and obsolete...
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Old March 13th, 2010, 10:45 AM   #5442
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Usually 55 - 65 mph in urban areas and 65 - 75 mph in rural areas, depending on state. Some sections in Texas and Utah allow 80 mph.

these are ridiculously low limits.

even the 80mph is low on low traffic sections, there should be 90 or 100mph...
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Old March 13th, 2010, 11:05 AM   #5443
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I doubt if Interstate cross slope and curvature is designed for such speeds. Imo, 75 - 80 mph is a fine speed limit in rural areas if it's not enforced too strictly.

What I do like about the US is the higher truck speed limits. Even the lowest speeds limits for trucks (California: 55 mph) are among the highest in Europe. Some areas even allow trucks doing 70 mph. I'm not sure if American rigs are more stable due to their much longer tractor base.
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Old March 13th, 2010, 03:33 PM   #5444
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Quote:
Originally Posted by ChrisZwolle View Post
I doubt if Interstate cross slope and curvature is designed for such speeds.
They aren't. The catalogue of available design speeds for Interstates maxes out at 70 MPH. Of course a state DOT or other highway agency can develop a compliant design which has enough "slack" to allow much higher speeds, but as a rule the driver can't count on more than 70 MPH in rural areas.

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What I do like about the US is the higher truck speed limits. Even the lowest speeds limits for trucks (California: 55 mph) are among the highest in Europe. Some areas even allow trucks doing 70 mph. I'm not sure if American rigs are more stable due to their much longer tractor base.
They aren't and they may in fact be more susceptible to jackknifing because the greater wheelbase of the tractor unit means more of a lever arm. Differential speed limits are unpopular with the trucking industry (especially owner-operators), and car drivers in general have a mixed opinion of them, so in the absence of clear evidence of their safety benefits, many states don't have them. FHWA several years ago sponsored a study to evaluate the safety implications of differential speed limits and was not able to prove that there were any benefits to having them.

This said, I have been told that a number of major trucking firms (Schneider, e.g.) voluntarily govern their tractor-trailers to speeds which are higher than the EU-wide limit of 56 MPH but still slightly below the speed limit for cars in many states--e.g., 62 MPH.
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Old March 13th, 2010, 10:19 PM   #5445
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Quote:
Originally Posted by ChrisZwolle View Post
I doubt if Interstate cross slope and curvature is designed for such speeds. Imo, 75 - 80 mph is a fine speed limit in rural areas if it's not enforced too strictly.

What I do like about the US is the higher truck speed limits. Even the lowest speeds limits for trucks (California: 55 mph) are among the highest in Europe. Some areas even allow trucks doing 70 mph. I'm not sure if American rigs are more stable due to their much longer tractor base.
I was surprised to see that in California. Having to drive up Interstate 5 between Los Angeles and Sacramento at 55mph must make a driver very sleeeeepy.
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Old March 13th, 2010, 10:30 PM   #5446
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Originally Posted by H123Laci View Post
these are ridiculously low limits.

even the 80mph is low on low traffic sections, there should be 90 or 100mph...
Its not that low. 100Mph is way too high for a lot of peoples cars. Its surprising how bad some cars drive over 70Mph
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Old March 13th, 2010, 11:35 PM   #5447
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Quote:
Originally Posted by ChrisZwolle View Post
What I do like about the US is the higher truck speed limits. Even the lowest speeds limits for trucks (California: 55 mph) are among the highest in Europe. Some areas even allow trucks doing 70 mph. I'm not sure if American rigs are more stable due to their much longer tractor base.
That's one of the things I don't like about our highways. Trucks (at least in Washington) often go as fast, if not faster than normal speed limits. With trucks as large as those in the U.S., and tight curves on the highways, it's almost unnerving to drive next to them when they're driving as fast as you are.

It's a safety issue too, because the distance it takes to stop an 18-wheeler is significantly higher. They often cruise in the middle lane as well, which creates havoc in the immediate traffic flow behind them.
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Old March 13th, 2010, 11:39 PM   #5448
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Originally Posted by J N Winkler View Post
They aren't. The catalogue of available design speeds for Interstates maxes out at 70 MPH.
I do remember seeing 80 mph design criteria somewhere from WSDOT...I'll see if I can find it.

Last edited by HAWC1506; March 13th, 2010 at 11:42 PM. Reason: Criteria = plural; criterion = singular. Doh!
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Old March 14th, 2010, 12:24 AM   #5449
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FAPG § 630(b) Supplement advises state DOTs (and other highway agencies performing federally funded construction) to put the important design criteria for a new-location highway, including the design speed, on the title page of the construction plans set. So if you find a title sheet that says 80 MPH, save it and show us--I can promise it will be very unusual.
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Old March 14th, 2010, 12:53 AM   #5450
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^ I work designing roads here in Canada (boring municipal ones though, not any interesting freeways or anything). The Canadian TAC manual has design criteria for design speeds up to 130km/h. In Canada, it is typical to design a road at 20km/h over what the posted speed limit will be once open to traffic. I would imagine design in the US has a similar factor of safety.
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Old March 14th, 2010, 01:14 AM   #5451
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The impression I have is that, pre-NMSL, posting the speed limit below the design speed was more common east of the Mississippi and the difference between the design speed and the posted speed was rarely more than 5 to 10 MPH. In the western states agencies typically designed for 70 and posted 70 in rural areas (yes, even Oregon). With the advent of speed limits up to 75 or even 80 MPH, there is now significant mileage which is posted in excess of the design speed. Building to a design speed 5 to 10 MPH above the speed proposed for posting is, again, happening largely east of the Mississippi, but in many cases the speed limits are increased (up to design speed or even beyond) when motorists' groups lobby for observance of the 85th percentile rule.

I have seen curve superelevation tables for design speeds of up to 100 MPH and 160 km/h, but I know of no Interstate highway which has been built to a design speed greater than 70 MPH.
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Old March 14th, 2010, 03:10 AM   #5452
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Quote:
Originally Posted by J N Winkler View Post
The impression I have is that, pre-NMSL, posting the speed limit below the design speed was more common east of the Mississippi and the difference between the design speed and the posted speed was rarely more than 5 to 10 MPH. In the western states agencies typically designed for 70 and posted 70 in rural areas (yes, even Oregon). With the advent of speed limits up to 75 or even 80 MPH, there is now significant mileage which is posted in excess of the design speed. Building to a design speed 5 to 10 MPH above the speed proposed for posting is, again, happening largely east of the Mississippi, but in many cases the speed limits are increased (up to design speed or even beyond) when motorists' groups lobby for observance of the 85th percentile rule.

I have seen curve superelevation tables for design speeds of up to 100 MPH and 160 km/h, but I know of no Interstate highway which has been built to a design speed greater than 70 MPH.
I've seen figures for 80 mph, but I have yet to see it be applied anywhere. So I guess the tables and such are there in many cases, but the application isn't.

For speed limits, the 85th percentile is used, so the speed at which 85 per cent of motorists are travelling will become the speed limit.

[Update]: found a diagram.



I'm too lazy to change it to the metric system. As someone who plans on studying civil engineering, the one thing I'm absolutely not looking forward to is using English system units. I can only hope that in the future, all of our roads will at least be designed in metric.

Maybe I'll move to Britain.

Last edited by HAWC1506; March 14th, 2010 at 03:39 AM.
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Old March 14th, 2010, 03:28 AM   #5453
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I'm surprised any transportation agency would permit a speed limit that is higher than the design speed of a given road. There are some pretty considerable legal ramifications posting a speed limit over the road design. If the speed limit is higher than the road design speed, the state would be liable for damages for virtually all traffic collisions.
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Old March 14th, 2010, 03:37 AM   #5454
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In Washington, there's actually a law stating that the maximum speed you can travel at is 70 mph. Any speed above that would require a change in legislation, which is not likely to happen anytime soon.

It's very difficult to generalize the U.S. when it comes to these criteria. Every state has a different system.
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Old March 14th, 2010, 04:47 AM   #5455
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This PDF comparing the WSDOT design manual to the 2004 AASHTO Green Book is also of interest:

http://www.wsdot.wa.gov/publications..._AASHTO-06.pdf

It appears that AASHTO now has a headline 80 MPH design speed for Interstates. The wrinkle here, however, is that the 2004 Green Book now allows lower k-values for summit curves (unwinding the dead-dog theory), so where vertical alignment is concerned today's 80 MPH could very well be yesterday's 70 MPH. I don't have my copies of the 1984 Green Book or the 1965 Blue Book to hand, so I can't comment in detail on how 70 and 80 MPH design speeds have been treated in AASHTO guidance in the past.

In any case, design speed is a bit of a red herring because different jurisdictions attribute different geometric characteristics to the same design speed. There are some reasons, not encountered in Europe, why American agencies need to be careful about speed limits in relation to design speed. First, American design standards generally allow curves to be built with smaller radii than is the case for equivalent design speeds in most European countries. Second, AASHTO standards give designers the discretion of choosing a maximum superelevation based on icing, and this is not linked to curve radius, so a curve of given radius at a given design speed can have multiple levels of side friction demand depending on the designer's choice of maximum superelevation. Third, the American approach to relating superelevation to curve radius is different and results in higher side friction demand as curve radius contracts. These factors, combined with the historical bias in favor of long-tangent, short-curve alignments, means that Interstates typically have less "slack" where higher speeds are concerned than European motorways.
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Old March 14th, 2010, 05:06 AM   #5456
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What's the speed limit in the USA when driving on motorways?
It varies, 70 mph is usually the standard. But it is acceptable to go 5 mph over the speed limit, so that would make 75 mph. But it depends where you are, some states allow you to go faster, paticulary in Texas and the southwest. In Texas you can go up to 85 mph on the Interstate.

Here is a full list by state.

http://www.motorists.org/speedlimits...d-limit-chart/
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Old March 14th, 2010, 05:30 AM   #5457
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I'm surprised any transportation agency would permit a speed limit that is higher than the design speed of a given road. There are some pretty considerable legal ramifications posting a speed limit over the road design. If the speed limit is higher than the road design speed, the state would be liable for damages for virtually all traffic collisions.
There is always an issue of German motorways with no speed limit. You can't say they were designed for an infinite speed. They just relay on driver's common sense. You drive as fast as you feel confident, it all depends from quality of your car and your abilities as a driver.
Even if you post speed limit lower than designed speed, if someone drives old crap car will have to go even slower to stay safe. You can't use design speed as a safe speed limit.
Also in Poland roads designed for speed of 110 or 120kph have speed limit posted 130kph if I'm correct.
Speed limit of, let say, 100mph on rural interstate wouldn't mean that you can safely drive 100mph in any car in any condition. If you mess around you can still be prosecuted for dangerous driving as you can in Germany.
I just personally think that blanket speed limit is quite blunt tool to improve safety, especially considering poor enforcement of the other rules on American roads (like lane discipline, indicating etc.)
Speed limit has more to do with legal system than with road design.

Last edited by geogregor; March 14th, 2010 at 05:36 AM.
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Old March 14th, 2010, 09:18 AM   #5458
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these are ridiculously low limits.

even the 80mph is low on low traffic sections, there should be 90 or 100mph...
Virtually nobody follows these exactly. In addition, in urban areas, these speed limits are very reasonable in the densest areas. With so many interchanges in succession in LA I get nervous going 70 in some parts simply from all the merging and splitting. I'm sure the roads back east that only have 2 lanes in each direction on many of the interstates don't have a choice but to slow down from the traffic.
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Old March 14th, 2010, 09:40 AM   #5459
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For what it's worth, the US Interstate system was designed for 70 mph speeds, which was the designated speed limit for Interstates in the late 50's, when the system came on line.

This does not include mountainous areas where speeds obviously need to be lowered.
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Old March 14th, 2010, 11:53 PM   #5460
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It varies, 70 mph is usually the standard
The funny thing is, I live in the US for 4 years, and I haven't seen any speed limit signs higher than 65... the fact that I travel only on the east coast may have something to do with it
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