Can anyone give examples of documents which suggest this practice? It seems not to be a good idea, but maybe there is some valid reasoning behind it.
I have attached herewith few chapters from a book which may help you to understand the concept better. Thank you
Hi Robert, I can´t see the logic, although design speed is theoretical. It is not possible to drive one car at two speeds simultaneously. In the Czech Republic design speed among others depends on the type of terrain (flat, hills, mountains,...). Worse terrain, lower design speed. Lower design speed means smaller horizontal radius. I think it is similar worldwide.
Veronika, I agree. However I just found one quote from Transit New Zealand which says
“It is good design practice to make the vertical alignment design speed 10 to 15 km/h greater than the horizontal alignment design speed to provide an additional safety margin (Transit New Zealand,2002)”
and in a discussion on LinkedIn an engineer said ".... I mean here is if the horizontal alignment is designed at 50mph and the vertical alignment is design for 60mph the effective design speed in this case would be 50 mph ...."
Hallo Narayan, thanks very much for the documents. I am downloading them now and will read them today.
It is confusing. But real speeds are usually higher than design speeds and from our research we know, that cars are able to drive horizontal curves much more quickly than design speed expect. Vertical alignment is limited more strictly with stopping sight distance. And risks with horizont are obvius. This could be the reason for the difference in speeds.
Maybe it has to do with centrifugal force; but from what little I can found written so far, using different values for speed for horizontal and vertical design sounds a bit like guessing in the dark. And what values do we take when we start detailed designing roads in 3 dimensions? I begin to wonder if the whole theory of speed in road geometric design is not very doubtful.
Hi Robert!
At first, Speed desig is different to operating speed. The first one is for desig purposes (theoretical speed), the second one is real speed. It is recommended that desig speed be greater than operational speed, this is for safety. In Colombia, design speed in vertical alignment is assigned equal to that corresponding to the horizontal alignment in the element of the road..
Hallo John but the question is, can (or should) you use different "design speeds" for vertical and horizontal alignment on the same section of road. You explain that, in Colombia, this is not done.
Hello !
I confirm: We use the same design speed for any elemet in the vertical and horizontal alignment (curve or tangent), and the design speed is defined by the horizontal alignment.
In Colombia we make vertical alignment design with design speed defined according with horizontal alignment's design speed of the element (curve or tangent). Now, we have some grades which the real operating speed is lower than the horizontal alignment.
Now I'm conducting an investigation about decreasing speed of vehicles according the grade of the road. If you have information about that, please send me those information.
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