Minimum Distance between vertical curves

[aladdin76]

Hi All

I have an Urban streets design of hilly area, the designer wanted to reduce the cost and followed the natural ground, with minimum cut and fill. As a result there are many vertical curves one after another. I couldn't find any Standard specification that determine the minimum distance between two vertical curves.

 

[bob3306]

There's no need for any separation as far as I know, and I've never seen any design criteria calling for one. Remember all vertical curves are in real-life are 25' long lengths of Tangent.

 

[PELS]

Whether or not you need distance between VC's or if you are able to use compound VC's should probably be dictated by local agency design standards, but in my experience, public safety issues (design speed, sight distance, etc.) have been important criteria in hilly terrain too. You state this is an "Urban street design" so I'm guessing the design speeds must be on the lower end.

I have seen a lot of high end custom home subdivisions where maintaining natural terrain and existing trees were goals to maintain the "charm" of the area. This dictated narrow streets, lower speeds and minimizing cuts & fills. The posted speed limits ended up being in the 15-20 mph range (or less in some cases) with restrictions on tall landscaping adjacent to roadways. In these designs there were no restrictions on compound VC's and in many instances there were combined horizontal and vertical curves.

So first check with the local jurisdiction requirements, discuss deviating from restrictive standards and then apply public safety criteria and also try to find examples of existing similar designs in your area.

 

[GoldDredger]

I've never heard of a minimum distance between horizontal curves. Frankly I've seen design examples of one horizontal curve connecting right into another at the point of tangency for both (one being crest and the other sag for example).

I have heard of mimimum lengths of vertical curves. Placer County CA for example, I think requires a mimimum 100-feet between BVC and EVC.

I think the most important factor for vertical curves is the horizontal sight distance. For crest vertical curves, the length would be dependent on design speed vs stopping sight distance (at what point can an object on the road be seen by a riders typical eye hieght)

For sag curves, the headlight sight distance and rider comfort control the design (according to my civil engineering reference manual).

Lastly, I think compound curves and spiral curves relate to horizontal curves, not vertical curves.

 

[PELS]

GoldDredger, you are correct about the terminology. When I was referring to a "Compound VC", I really meant an "Unsymettrical VC", which is really two vertical curves of different lengths that are tangent to each other.

 

[gusongjun]

Horizontal curves usually connected by tangential straight lines. The length of tangential lines shall be not less than 4 times design speed. For example, design speed is 60 km/h, the minimum length of tangential line shall be 240 m.

Regarding spacing vertical curves, I can’t remember any specification which required minimum spacing.