Intersection on vertical curve (crest)
Intersection on vertical curve (crest)
(OP)
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
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





RE: Intersection on vertical curve (crest)
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.
RE: Intersection on vertical curve (crest)
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
RE: Intersection on vertical curve (crest)
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.
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"...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
RE: Intersection on vertical curve (crest)
RE: Intersection on vertical curve (crest)
RE: Intersection on vertical curve (crest)
I'm curious: what are your grades in and out of this crest?
RE: Intersection on vertical curve (crest)
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