Should highways in desert states (CA, NV, AZ, NM, TX, etc) have speed limits?

[Roadman66]

Do you think it is necessary to post speed limits on roads that travel through desert land forms? Even though I see a lot of roads that are 55-65 mph in deserts, should roads be 90, 95, or even 100? There is no civilization at all over there. Are there any two lane roads that are higher than 65 mph?

[Brandon]

I think a black on yellow advisory limit may be more appropriate, IMHO.  I've thought this for freeways for quite some time now.  Such as:

AUTO
SPEED
70
M.P.H.

Black on yellow with this:

TRUCK
SPEED
70
M.P.H.

Black on white.

[NE2]

Are there any two lane roads that are higher than 65 mph?

85 mph handles one lane Interstate 366.

[vtk]

I think something like this would work:

Black on white standard speed limit sign:
SPEED LIMIT 120

Then every 5 miles or so, black on yellow diamond:
WATCH FOR STOPPED VEHICLES ON SHOULDER or WATCH FOR SLOWER TRAFFIC
plus black on yellow rectangle:
90 MPH

Of course, every curve and hill, no matter how gentle, must be evaluated for sight distance and G forces, and a maximum safe speed determined and posted as an advisory speed below an appropriate diamond warning sign – though I suppose if that speed is absurdly high (like twice the regulatory speed limit or more) then the advisory plaque can be omitted.

[Brandon]

Are there any two lane roads that are higher than 65 mph?

85 mph handles one lane Interstate 366.

This isn't even funny anymore.

[roadfro]

Roads aren't typically engineered for extremely high speeds over the 80s or so. Posting speed limits higher than that is not a good idea unless the roads are redesigned to accommodate it...especially where roads curve or go through mountains.

The assertion that there is no civilization in these desert areas is exaggerated. Granted, populated towns are more spread out, but saying for example that there's nothing in the desert between Las Vegas and Northern Nevada is inaccurate...

Other threads have answered the speed limit question...

[Alps]

The 12-foot lane width is based on operating speeds in the 50-80 mph range. The faster you go, the narrower the lane feels because each input is magnified. So at very high speed limits, you may want to consider wider roads. The other issue is designing curve superelevation. How fast or slow do you want them to be? You can bank a gradual curve for 100 mph but it will be difficult to stay on it at 60 mph.

[vtk]

Roads aren't typically engineered for extremely high speeds over the 80s or so. Posting speed limits higher than that is not a good idea unless the roads are redesigned to accommodate it...especially where roads curve or go through mountains.

That's exactly my point.  Set the regulatory speed limit to something that pragmatic cops can "agree" is the threshold of recklessness in the absence of any specific hazards or design deficiencies, then analyze the road's geometry to determine exactly where specific hazards or design deficiencies exist that would call for slower speeds, then sign those specific hazards or design deficiencies along with appropriate advisory speeds.

[corco]

Set the regulatory speed limit to something that pragmatic cops can "agree" is the threshold of recklessness in the absence of any specific hazards or design deficiencies, then analyze the road's geometry to determine exactly where specific hazards or design deficiencies exist that would call for slower speeds, then sign those specific hazards or design deficiencies along with appropriate advisory speeds.

Wait, so you're saying you want people to drive as fast as possible on straightaways and slow down around corners? You're crazy.

[vtk]

Set the regulatory speed limit to something that pragmatic cops can "agree" is the threshold of recklessness in the absence of any specific hazards or design deficiencies, then analyze the road's geometry to determine exactly where specific hazards or design deficiencies exist that would call for slower speeds, then sign those specific hazards or design deficiencies along with appropriate advisory speeds.

Wait, so you're saying you want people to drive as fast as possible on straightaways and slow down around corners? You're crazy.

If the driver wants to get through the area as fast as safety permits, that's exactly the way to do it.  Drivers more concerned with fuel efficiency can make their accelerations and decelerations gradually, or stay at a relatively constant (slower) speed if they want.

Really, this is just an extrapolation of the way things are (or at least, should be) done on roads with 55 MPH or similar speed limits, but recognizing that significantly higher speeds can be safe under certain circumstances.

[corco]

No, that won't work. It's too logical.

[1995hoo]

To my mind the correct question isn't so much "should highways in desert states have speed limits" but instead "how should states set speed limits, and above what number should there be only a recommended speed limit like on the autobahn?"

In other words, even desert states have urban areas or twisty roads where some kind of speed limit will be appropriate. Consider that even an empty state like Nevada has Las Vegas, a good-sized city. They're never going to abolish all speed limits. But since they also have wide-open roads in the middle of nowhere, I've always thought that the approach of "state legislature sets a maximum number and the DOT can adjust it" is a poor approach. Among other reasons, it artificially caps limits based on arbitrary factors–for example, cars built today are immensely safer and better-handling than cars built in the 1960s and early 1970s with their poor brakes, skinny tires, etc. There are comparatively few cars from that era still on the road. To suggest that the highest speed limit in effect back then based on the cars then on the road is inherently the maximum safe speed limit today doesn't strike me as at all logical in most cases (setting aside, for discussion purposes, the perfectly valid proposition that things like increased congestion on some roads will work against higher speed limits in some places). There's nothing inherently wrong with speeds creeping up as cars improve.

I suppose that could be a topic for the Off Topic board–cars you've owned over the years.


One thought on desert highways that comes to mind, though, is whether the mirage effect you get on hot days might pose a hazard if you keep increasing the speed limit. I've never driven in the desert so I don't know, but certainly most of us have experienced that effect during the summer at some point. On two-lane roads, where you pass in the oncoming lane, the mirage effect seems like it would pose an issue, especially because so many American drivers do not turn on their headlights on those roads unless they have DRLs, and so it seems like a lower speed limit might be prudent in areas where this is a particular problem. On a multiple-lane road it would seem to be less of a problem.

[corco]

One thought on desert highways that comes to mind, though, is whether the mirage effect you get on hot days might pose a hazard if you keep increasing the speed limit.

Passing under mirage circumstances can be pretty unnerving- when I'm driving "back east" (Eastern Nebraska!-  hey, a fun exercise might be for everyone to draw a mental map of the United States and post it) I find the visibility distance I need to be comfortable passing isn't nearly as great as it is in the west.

[vdeane]

Roads aren't typically engineered for extremely high speeds over the 80s or so. Posting speed limits higher than that is not a good idea unless the roads are redesigned to accommodate it...especially where roads curve or go through mountains.

The assertion that there is no civilization in these desert areas is exaggerated. Granted, populated towns are more spread out, but saying for example that there's nothing in the desert between Las Vegas and Northern Nevada is inaccurate...

Other threads have answered the speed limit question...

The interstates in the 1960s were designed for an average speed of 70mph.  That's AVERAGE speed, meaning that these roads were designed with 60s standards, for 60s cars, with the assumption that about half of traffic would be going faster than 70.  I'm sure a modern interstate could handle faster.

[J N Winkler]

The 12-foot lane width is based on operating speeds in the 50-80 mph range. The faster you go, the narrower the lane feels because each input is magnified. So at very high speed limits, you may want to consider wider roads.

Historically speaking, I am not aware that the input magnification effect has been a consideration in choosing unit lane width for freeways or other high-speed roads.  My understanding is that unit lane width, in most countries, has been driven by four factors:

*  What other countries are doing (inferred from published standards, reports of technical characteristics of new major roads, etc.)

*  What construction and use regulations prescribe for large vehicles

*  What lane width is sufficiently large that vehicles will not alter lane position in response to another vehicle moving in an adjacent lane (with lane widths as low as 8', for example, you see vehicles moving to the side or even out of the lane altogether to leave additional room for a passing vehicle)

*  What unit lane width seems safest in terms of accident experience (Ezra Hauer has noted that the bottom of the valley in terms of accidents falls around an unit lane width of 11.4', with 12' being actually slightly less safe than 11')

12' (or its metric equivalent) has been pretty much universal in motorway design since the Reichsautobahnen began to be constructed in 1933.  The original design catalogue provided for design speeds of up to 160 km/h in flat country (with some nomographs providing for superelevation at speeds of up to 170 km/h) but the unit lane width was the equivalent of 12' for all design speed classes.  British highway engineers in the 1930's had access to curve tables providing for design speeds of up to 100 MPH, but the prescribed unit width was 11' on roads expected to handle significant volumes of lorry traffic.  This was moved up to 12' in the early 1950's (before the first motorways were constructed) partly on the strength of Road Research Laboratory studies into lorry lane positioning.

The other issue is designing curve superelevation. How fast or slow do you want them to be? You can bank a gradual curve for 100 mph but it will be difficult to stay on it at 60 mph.

Actually, the really important variable is curve radius.  For any design speed, it is the curve radius that determines how forgiving the curve is to traffic moving at a significant speed differential relative to the adopted design speed.  The higher the curve radius, the less superelevation that has to be provided for the given design speed, and the less lateral acceleration the driver feels when he or she drives a fixed increment above the design speed.  The curve with minimum radius and maximum superelevation for the adopted design speed is always the least forgiving to traffic deviating from that design speed.

With the exception of glare ice and other low-friction conditions, I don't think traffic driving significantly below the design speed needs to be a prime consideration.  Curiosities like the Sale-Meknès military road, built by the French in Morocco in the 1920's with 20% superelevation in the middle and 5% superelevation out to the sides, are just those--curiosities.

There is nothing conceptually wrong with having a derestriction policy, particularly if it is confined to freeways passing through areas with very low population density.  However, in this country we have a tradition of strict speed enforcement which has inculcated in drivers the expectation that roads will be comfortable and safe to drive at the posted speed limit in fair-weather conditions.  Thus, we would need to post a suggested maximum speed in order to give drivers a reasonable amount of advance notice as to the geometric design characteristics of the road.  As in Germany, this could be linked to a presumption of civil and criminal liability attaching to drivers who experience accidents while driving above that suggested maximum.

We would still need to retain speed limits going through cities (or, more precisely, areas with closely spaced exits carrying significant volumes of entering and exiting traffic), on the approaches to tollbooths, and in other conditions where a full stop is either necessary or reasonably likely.

[empirestate]

Do you think it is necessary to post speed limits on roads that travel through desert land forms? Even though I see a lot of roads that are 55-65 mph in deserts, should roads be 90, 95, or even 100? There is no civilization at all over there.

Phoenix might disagree with you there...

[J N Winkler]

The interstates in the 1960s were designed for an average speed of 70mph.  That's AVERAGE speed, meaning that these roads were designed with 60s standards, for 60s cars, with the assumption that about half of traffic would be going faster than 70.  I'm sure a modern interstate could handle faster.

Nope.  That statement actually reflects a misunderstanding of the concept of design speed.  Design speed, under the definition traditionally used, is the upper bound of the speeds most drivers adopt when conditions are so favorable that speed choice is governed entirely by the geometric design characteristics of the road.  Design speed is not an average of the speeds such drivers would choose.  The attribution of geometric design elements to a particular design speed is based on gross measurements that have not changed much since the 1930's--driver's eye height above the road, driver's reaction time, friction achievable between tires and the road surface, maximum lateral acceleration the average driver is prepared to tolerate, etc.  The main effect of the automotive engineering improvements of the last 70 years has been to make it more comfortable and secure to cruise at high speed, not to shift choice of design speed upward or to encourage designers to make more or less conservative choices of design elements for a given design speed.  As a consequence, the changes in design of cars have promoted only a very limited amount of "speed creep."

As a general rule of thumb, any rural Interstate you are likely to drive on, regardless of age of construction, will have been built with a 70 MPH design speed.  The first set of Interstate design standards, published in 1943, envisioned a maximum design speed (to be used in flat terrain) of 75 MPH, but I cannot remember having seen a set of construction plans (out of the thousands I have looked at just for US states) for a highway designed for 75 MPH.  It is true that the elements associated with a particular design speed have changed in time, but those changes have uniformly been part of a convergence toward current standards which was essentially complete by 1965.  There is much less difference between an Interstate built in 1965 and an Interstate built in 2005 than there is between an Interstate built in 1955 and an Interstate built in 1965.  Nowadays we pay more attention to questions of design consistency, but the biggest changes between 1965 and 2005 have related primarily to clear zones and roadside protection (standards for which still diverge considerably among states).

[BigMattFromTexas]

Are there any two lane roads that are higher than 65 mph?

Pretty much every two lane road in Texas has a 70 mph speed limit... On U.S. 90 north of Del Rio, the speed limit is 75, and on U.S. 385 around Big Bend Nat'l Park, it's 75..
BigMatt

[vdeane]

The interstates in the 1960s were designed for an average speed of 70mph.  That's AVERAGE speed, meaning that these roads were designed with 60s standards, for 60s cars, with the assumption that about half of traffic would be going faster than 70.  I'm sure a modern interstate could handle faster.

Nope.  That statement actually reflects a misunderstanding of the concept of design speed.  Design speed, under the definition traditionally used, is the upper bound of the speeds most drivers adopt when conditions are so favorable that speed choice is governed entirely by the geometric design characteristics of the road.  Design speed is not an average of the speeds such drivers would choose.  The attribution of geometric design elements to a particular design speed is based on gross measurements that have not changed much since the 1930's--driver's eye height above the road, driver's reaction time, friction achievable between tires and the road surface, maximum lateral acceleration the average driver is prepared to tolerate, etc.  The main effect of the automotive engineering improvements of the last 70 years has been to make it more comfortable and secure to cruise at high speed, not to shift choice of design speed upward or to encourage designers to make more or less conservative choices of design elements for a given design speed.  As a consequence, the changes in design of cars have promoted only a very limited amount of "speed creep."

As a general rule of thumb, any rural Interstate you are likely to drive on, regardless of age of construction, will have been built with a 70 MPH design speed.  The first set of Interstate design standards, published in 1943, envisioned a maximum design speed (to be used in flat terrain) of 75 MPH, but I cannot remember having seen a set of construction plans (out of the thousands I have looked at just for US states) for a highway designed for 75 MPH.  It is true that the elements associated with a particular design speed have changed in time, but those changes have uniformly been part of a convergence toward current standards which was essentially complete by 1965.  There is much less difference between an Interstate built in 1965 and an Interstate built in 2005 than there is between an Interstate built in 1955 and an Interstate built in 1965.  Nowadays we pay more attention to questions of design consistency, but the biggest changes between 1965 and 2005 have related primarily to clear zones and roadside protection (standards for which still diverge considerably among states).

Maybe the politicians should realize that drivers don't like to feel "comfortable".  The reason most people speed is because driving the speed limit feels like you're moving at a snail's pace.

[jwolfer]

...... Consider that even an empty state like Arizona has Las Vegas, a good-sized city.....

You moved Las Vegas into another state

[1995hoo]

...... Consider that even an empty state like Arizona has Las Vegas, a good-sized city.....

You moved Las Vegas into another state

D'oh. Thanks. I've gone back and fixed it. I was thinking Nevada but typed Arizona, perhaps because I originally thought of Phoenix as an example of a major city in a desert state but I then changed my mind because Nevada's supposed to be emptier.

[on_wisconsin]

Its not a desert state but I-90 though rural South Dakota shouldn't have a speed limit. (The same could be said for I-94 in ND.)

[J N Winkler]

Maybe the politicians should realize that drivers don't like to feel "comfortable".  The reason most people speed is because driving the speed limit feels like you're moving at a snail's pace.

I don't agree.  Perhaps our perceptions differ because Kansas and upstate New York have different approaches to setting speed limits, but I generally drive the speed limit around here (except when overtaking) and I am not overtaken that often.  I feel comfortable driving at the speed limit in fair weather and I don't want to drive at a speed high enough to cause discomfort either to myself or my passengers.

On the urban freeways around here (which generally have one-mile interchange spacing on their busiest lengths with speed limits of 60 or 65 MPH), I have noticed a tendency for speeds to cluster tightly within 2.5 MPH on either side of the posted speed limit, but I would say that the 85th percentile speed is rarely if ever greater than 5 MPH over the speed limit.  This suggests that the speed limits are appropriately posted and the vast majority of drivers are comfortable with them.

If the speed limits were still posted at 55 MPH, as they were in the mid-1990's before the NMSL was abolished, then yes, it would be a different story.  But that culture of underposting has been in decline even on the east coast.

[roadman65]

Can the human ego handle high speed limits anymore? I see people hitting 100 on I-4 through Orlando, FL where it is only 50 mph not even 55.  Heck I see people on off interstates traveling fast into a red signal before putting on the brakes and giving Midas extra business for their brake wear.

[Duke87]

I don't want to drive at a speed high enough to cause discomfort either to myself or my passengers.

I do tend to drive slower when I have someone else in the car with me. But that isn't just due to additional concern with not wanting to make them uncomfortable. It's also because having passengers in the car is a distraction. Need to stay 100% focused on the road to drive like a maniac, can't do that with a passenger.

I don't know how much other people experience this, but I am a lot more comfortable with speed and also particularly with other obnoxious behavior when I'm behind the wheel. When I'm driving by myself on the freeway if space permits I may start changing lanes with some frequency to keep passing people on either side, and I'm completely comfortable with it so long as I'm in a groove. But if I am a passenger in a car with someone else doing this, it freaks me out. Why? Well, being in control versus not being in control makes all the difference. I'm in general always more comfortable with anything when I am in control.