Vertical Curves - Sight distance

I am no help on this. I abhor vertical curves and refuse to work them because I truly hate the quadratic equation. 🙂 I break out in cold sweats thinking back to Algebra II on the matter.

Good luck.

Thanks Vern, so no sight distance value for the sag. It seems that sag vertical curves have a special property regarding sight distance that I have not quite entirely grasped. The formulas tend to focus on solving the required length of curve for a given K value, rather than solving the sight distance.

Could anybody enlighten me?

imaudigger, post: 359635, member: 7286 wrote: Thanks Vern, so no sight distance value for the sag. It seems that sag vertical curves have a special property regarding sight distance that I have not quite entirely grasped. The formulas tend to focus on solving the required length of curve for a given K value, rather than solving the sight distance.

Could anybody enlighten me?

Basically, that is because where ever you are on the curve you are looking over any object that may be on line. Note that none of the curves posted meet AASHTO K value standards.

Vern, I was under the impression that you used the headlight sight distance formula for sag curves.
I guess maybe with the min. K values provided, headlight sight distance isn't an issue?

100,171.35,57.94
220,319.07,60.65
80,135.51,36.48

imaudigger, post: 359643, member: 7286 wrote: Vern, I was under the impression that you used the headlight sight distance formula for sag curves.
I guess maybe with the min. K values provided, headlight sight distance isn't an issue?

Take a look at the data. You don't have 3.5 feet difference from your low point to high point. Just my personal opinion is that vertical curves are barely required along that data set.

navaran, post: 359648, member: 1096 wrote: 100,171.35,57.94
220,319.07,60.65
80,135.51,36.48

What program or formulae are you using? I can't reproduce any of those numbers nor any of [USER=7286]@imaudigger[/USER] answers.

navaran, post: 359648, member: 1096 wrote: 100,171.35,57.94
220,319.07,60.65
80,135.51,36.48

The K values are similar, but the sight distance values are significantly different that what I was expecting.
My gut instinct says they are too short given the flat grades.

For example using the first crest vertical curve..

Where S>L
L = 100.00'
h1 = 3.50'
h2 = 2.00'
A = 1.73%

I get a sight distance of 673.79'. My software is saying 674.94'. I'll buy that (although I am not sure why it doesn't match). That is significantly different than the data you provided.
Of course that is making the assumption that there is no grade changes further down the profile within that 674'

Hmm....

Vern, I understand what you are saying. I am mainly concerned with figuring out the formulas in order to check the software reports (for future reference).

Then with the 220' sag vertical curve...
Where S>L
L = 220'
h1 = 3.5'
h2 = 2.0'
A = 3.63%
Angle of headlight = 1å¡

I'm getting a sight distance of 397.51'

The software is telling me it is 319.01' (which agrees with Navaran's report) so it must be using a different formula.

I think I goofed the last calc.

I requested that Carlson furnish the formulas that it uses to calculate the sight distances.
I will re-visit the issue once I have more information.

Sag curve design can be affected by three factors:

• Headlight sight distance is important for roads with lots of crests and valleys, and if the design speed is high.
• Comfort distance can be important. You don't want the vehicles to "bounce" at the bottom of a sag.
• Drainage can be an issue if the profile is generally flat. Very long sag curves can create areas without enough grade for water to flow, leading to problems with icing.

The K value is basically the distance in feet it takes to get a 1% change in grade.

If you have -2% coming in and +2% going out, 4% total, a 400 foot length would give you a K value of 100.
200 foot length would give you a K of 50. If you want a K of 75 then 75x4= 300 foot length

Can't help with the sight distances.
James

This is what started this whole investigation.
My first issue was that the software didn't specify what type of sight distance it was reporting nor the heights used.

Dirt is cheap, just raise that vpi at 54+82 about a foot. :stakeout:

edit: that 11.11% should not be there.

James

JaRo, post: 359701, member: 292 wrote: Dirt is cheap, just raise that vpi at 54+82 about a foot. :stakeout:

edit: that 11.11% should not be there.

James

My point was that the sight distance and K values don't agree from one report to the next.
-------
"Dirt is cheap"...Tell that to the permit folks...the fill is going in a stream bed.

My smarta$$ comment was after looking at the first drawing. Just being funny.

After looking at the second report, there is a point that should not be there. That station 54+91 may be the low point of the sag but the software is figuring the slope from the VPC to the VPI to the sag 8.92 feet away and then to the VPT. Thats where the 16.83 K value is coming from and it's just wrong. That is where the comment about the 11.11% came from.

Not being familiar with Carlson, I'm at a disadvantage, but that point should not be in that report or at least the software should not be calculating a distance and slope to it.

James

JaRo, post: 359718, member: 292 wrote: My smarta$$ comment was after looking at the first drawing. Just being funny.

After looking at the second report, there is a point that should not be there. That station 54+91 may be the low point of the sag but the software is figuring the slope from the VPC to the VPI to the sag 8.92 feet away and then to the VPT. Thats where the 16.83 K value is coming from and it's just wrong. That is where the comment about the 11.11% came from.

Not being familiar with Carlson, I'm at a disadvantage, but that point should not be in that report or at least the software should not be calculating a distance and slope to it.

James

I'm immune to SmartA$$ remarks, so no problem there at all.

You are right on the money with the report using the sag station.
Turning that report option off changes the results.
Too many profile revisions and reports..my brain is spinning.