advisory signs

[zachary_amaryllis]

i live on a rather curvy mountain road, and it seems the 'advisory' signs are posted artificially low, at least for a car. not that i habitually drive like a maniac, but i've found i can go 10-15 over the advisory, and it really doesn't feel too fast (not pressed up against the side of the car, no screeching tires etc.)

is it... like a 'one speed fits all' advisory (a large truck might not be able to take the curves at 15 over)?
i drive the whole route from town to my house, the speed limit is 45 for the first few miles, then drops to 35. i normally go 45-50 the whole way, and never have to use my brakes, just let off the gas for the curves and i'm fine.

or am i a road hazard?

[thspfc]

Those signs are always on the cautious side of things, and rightfully so IMO. People always go 10-15 over the regualr speed limit, so obviously if the advisory sign was posted at or close the maximum speed you could take the curve at, people would think they could go 10-15 faster than that, which would result in cars flying off cliffs.

[Max Rockatansky]

I always assumed most advisory signs were oriented to how fast a freight vehicle ought to take curves.  One of the worst had to be Arizona.  Some of the advisory speeds (and actual limits) are downright nanny-like.  AZ 89A in particular comes to mind in Oak Creek Canyon and south of Jerome. 

[corco]

Oregon actually recalibrated their curve speeds a couple years ago - the original speeds were based on how well a 1952 Packard or some shit (using a ball suspended in water - somewhat like a level - to determine how steep the curve was) could safely handle a curve and were hilariously underposted - to the point that you could routinely go 20-30 over the curve speeds.

Here's an article and the associated study that has some insight into how these are calculated:

https://www.statesmanjournal.com/story/news/2016/05/06/speeds-increase-curve-advisories-oregon-roads/84023134/
https://www.oregon.gov/ODOT/Programs/ResearchDocuments/SPR_641.pdf

[GenExpwy]

I wish New York would re-check its curve advisories. This curve posted for 40 really should be taken at 40, while this one posted for 25 – many decades ago, using who-knows-what criteria – could be easily taken at 55.

[zachary_amaryllis]

Here's an article and the associated study that has some insight into how these are calculated:
https://www.statesmanjournal.com/story/news/2016/05/06/speeds-increase-curve-advisories-oregon-roads/84023134/
https://www.oregon.gov/ODOT/Programs/ResearchDocuments/SPR_641.pdf

this was quite interesting and helpful... thank you
now i don't feel like such a road-psycho.

[JoePCool14]

I agree with thspfc. You treat them similarly to your average speed limits, in that they are set 5-10 mph lower than you could necessarily go.

As for another factor, if you are unfamiliar with a road, you're probably going to take it a bit easier on the gas since you're not sure what to expect. I've recently been driving on rural roads around my school, and for the first few times, I didn't always remember each curve and how fast was too fast for them. Once you've driven a road 5, 10, 50, 100, 1000 times, you're obviously going to get more comfortable driving it. Advisory speeds are most useful for unfamiliar drivers.

[skluth]

I'll usually drive the advised speed if I'm not familiar with a road, especially when there's a severe drop-off on one side like many of California's mountain roads or Missouri's Ozarks. But yes, I've usually found them low and will speed through at whatever speed I feel safe driving after having driven it a couple times. It seems the curves with severe reduced speeds, like dropping from 55 to 25 mph, are closer to reality and if it says something like 15 mph it's definitely worth hitting the brakes and taking it at close to 15 mph.

[kphoger]

I've found that it varies by state.  For example, Missouri posts quite reasonable speed advisories, and I don't recommend going more than about 5 over that.

[hbelkins]

It's been my experience that most advisory speeds are posted at least 10 mph slower than you can safely take the curve.

Advisory speeds Signs are most useful for unfamiliar drivers.

FIFY.

If you're familiar with a road, you don't need to see signs to know where curves are, what the road name or number is, or where it goes. I don't pay attention to signage on roads with which I'm very well acquainted, unless the sign is a new sign that I haven't see before. If it's been a few weeks or months since I've been on a particular stretch of road, and see a "Bump" or "Break In Pavement" sign, I know to slow down and watch for the hazard.

[wxfree]

The TxDOT speed zone manual tells us some things about this.  The old method to establish a curve advisory speed was with a ball bank indicator.  That has a ball in a smile-shaped curved tube.  When the car is at rest, or moving straight, the ball will sink to the bottom, in the middle.  Going around a curve, the ball will roll up one side.  If you go around the curve faster, it will roll farther up that side.  The tube is marked in degrees or something, and a certain reading was considered to be the safe limit.  This tends to produce low advisory speeds.  TxDOT no longer approves of the method.

There are now three approved methods.  The first is the direct method, which is based on recording the speeds people drive, similar to the way speed limits are (should be) set.  The second method is based on GPS.  The curve radius and deflection angle are estimated by GPS, superelevation is measured or estimated, and a speed is calculated.  The third method is based on the design of the curve, and can be used only if the precise plans are still available.  A computer determines the appropriate speed.

I've noticed that some advisory speeds were increased after the new methods started being used, but in my experience most were not.  I don't know if that's because they're considered appropriate and won't be changed or if they just weren't reanalyzed.  They might change when the road is rebuilt and the new construction plans can be used for new calculations.

[hbelkins]

Kentucky ball-banks superelevation on curves, bridges, and ramps. I don't know if we ball-bank flat curves to determine advisory speeds or not.

[doorknob60]

Oregon actually recalibrated their curve speeds a couple years ago - the original speeds were based on how well a 1952 Packard or some shit (using a ball suspended in water - somewhat like a level - to determine how steep the curve was) could safely handle a curve and were hilariously underposted - to the point that you could routinely go 20-30 over the curve speeds.

Here's an article and the associated study that has some insight into how these are calculated:

https://www.statesmanjournal.com/story/news/2016/05/06/speeds-increase-curve-advisories-oregon-roads/84023134/
https://www.oregon.gov/ODOT/Programs/ResearchDocuments/SPR_641.pdf

Yeah, if I'm remembering a lot of them were switched out around the same time that the speed limit on eastern Oregon highways was raised (to 70 or 65 depending on highway). Many of the advisory speeds were replaced with speeds 10-15 MPH higher, with no change to the actual road.

Here's one example of many, I-84 next to La Grande, raised from 50 to 65 (change the GSV date): https://www.google.com/maps/@45.3452399,-118.1229908,3a,75y,312.35h,87.26t/data=!3m7!1e1!3m5!1sue4IAvYhpuKBJSJ-T_IJsg!2e0!5s20120501T000000!7i13312!8i6656 (for reference, the actual speed limit was raised from 65 to 70 in this location)

The confusing thing, is some roads never got new advisory speeds, and some have a mix of new and old signs. So on some curves you can comfortably fly through the curves at 20 above the advisory, and some roads I wouldn't push it past 5-10 above. Cabbage Hill west of Pendleton, has some curves signed for 65 and some signed for 45, and there's not a huge difference between them (as they used to be 50 and 45, respectively).

Inconsistency is not just an Oregon problem, here is I-184 at I-84 in Idaho. There's a curve signed for 40, and then shortly after, another one signed for 55. Maybe I'm crazy, but both curves seem pretty similar to me (other than how drawn out they are), and I feel comfortable with both at around 55-60 MPH (I wouldn't feel comfortable pushing the 2nd one to 70 for example). https://www.google.com/maps/@43.599366,-116.2875953,3a,75y,247.67h,76.64t/data=!3m6!1e1!3m4!1s9-53GBtsfRIO4eU3y3b6hw!2e0!7i16384!8i8192

[CardInLex]

Kentucky ball-banks superelevation on curves, bridges, and ramps. I don't know if we ball-bank flat curves to determine advisory speeds or not.

We do.