Intersection on vertical curve (crest) 1

[nutbutter]

Hello,
I have a question. I'm just laying out some conceptual plans for a residential roadway. I don't have much experience with roadway design.

My initial layout was designed so that their was an intersection smack dab in the middle of a vertical crest curve. My intuition tells me that this wouldn't be ideal because site distances would be limited for cars turning onto the street from the side street. It seems to me that they would be looking downhill in both directions and wouldn't have the visibility that they would have if it were flat. Is this correct?

A drafter I work with who has more experience was saying that it's better to have an intersection at a vertical crest curve. He seemed to believe that you would be able to see further since you were looking downhill. I disagree.

Can anyone clear this up for me?

Thanks,
Steve

 

[proletariat]

I think the drafter is right, but looking uphill or downhill has nothing to do with it. The biggest impedement to sight distance is going to be the crest of the roadway. If you situate you intersecting street on the crest, then drivers leaving the intersecting street will be able to see to their right and left for approaching traffic. Drivers on the intersected street will be able to see traffic approaching from the opposite direction, to make a left turn, and drivers will be able to see cars stopped to make right or left turns without rear-ending them.

If you situated your intersecting street to either side of the crest, the crest itself could impede sight distance. Of course, none of this matters if your curve is long enough.

 

[nutbutter]

Okay. I see what you're saying. The vertical crest curve will make little difference if the curve length is long enough. However, I'm still not convinced that the crest curve is a better spot for an intersection than a flat area. I just don't understand how it allows you to see further.

If one were to stand at the crest, it would seem that at some certain distance his view would be blocked by the curve of the roadway itself (he wouldn't be able to see cars approaching beyond this point). It would seem that a flat area would offer the greatest sight distance because there is no curvature of the roadway to impede distance. I could be entirely wrong.

Thanks for the input.

Peace,
Steve

 

[ACtrafficengr]

Why don't you plot out your profile and measure your sight distance at various potential intersection locations?

You are right in that the best place for an intersection is on a long vertical tangent - either flat or a steady grade with no crests.

Depending on the profile of the road, sometimes the second best location is at the crest, since drivers will be able to see over the crest to some extent.

Usually the worst choice is just below the crest, because the curve will critically limit sight distance in one direction. In fact, research has shown that crashes are rare at crest curves except when they obscure an intersection or driveway.

One more consideration: what is the profile of the intersecting road? If it falls off, can you provide a near-level "platform" for vehicles waiting to turn? If not, take the resulting lower driver's eye height into account when measuring sight distance, designing guiderail or landscaping, etc.

------------------------------------------
"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail."

Wm. Phelps Eno, ca. 1928

 

[Maury]

The view from the side street might be obstructed by the curve and it might not, depending on the length of the curve and the rate of curvature (K). Many design handbooks have formulas for calculating the passing sigth distance, but try drawing a profile of the alignment and lay a straight edge on it. It's OK to use differing horizontal and vertical scales. The height of the driver's eye and the oncoming vehicle should be considered as 3.5' above the roadwau surface. If the side road is truly at the summit, I think the sight distance will be good.

 

[jeremyk55]

You will also need to look at the horizontal sight distance triangles for looking to the right and left. Since this is for a residential development, keep in mind that signs, trees, and landscaping can obstruct the view of the driver on the side road. Some agencies will require the entire line of sight to be inside the right of way.

 

[TerryScan]

Steve,

I'm curious: what are your grades in and out of this crest?

 

[nutbutter]

G1= 10.00%
G2= -8.75%
L= 375'

K=20 as required by the city. it's a 25 mph local residential street for a small development.

take care,
steve