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Ambrug20

Project Engineer
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I came a cross the problem (Test Master #27 transportation): see attached file.

Why do we have to find percentage of clear zone and why we use 14’/20’

and 14’ (not 16’)/22’.

Why don’t we use data from the table? What is that data any way in this case?

Did I puzzle enough? :bananapowerslide:

Clear_Zone.pdf

 

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  • Clear_Zone.pdf
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I came a cross the problem (Test Master #27 transportation): see attached file.Why do we have to find percentage of clear zone and why we use 14’/20’

and 14’ (not 16’)/22’.

Why don’t we use data from the table? What is that data any way in this case?

Did I puzzle enough? :bananapowerslide:

Tanya,

While your question was a little confusing, I think I know what you are asking. It looks like your problem #27 (attached .pdf) is referencing the 2002 AASHTO RDG and I am using the 2006, but I don't think it matters. This was a fun exercise, thanks. :)

I know what you are thinking - why can't you simply go to Table 3.1 and get the Foreslope and Backslope Clear-zone distances directly from the table? The reason is because there is a V ditch in the clear-zone. This now directs you to Figure 3.6. Plot your Foreslope slope and your Backslope slope on the figure - if your plotted point falls outside of the shaded area (the Preferred channel cross slope), you need to solve for an adjusted clear-zone (you can't simply use Table 3.1 directly).

Please see Example G, page 3-26 (2006 RDG). It's a perfect example, very similar to your problem #27.

Step 1.

14'/20' = 70% (14' = edge of traveled way to ditch FL and 20' = lower range of foreslope Clear-zone from Table 3.1)

14'/22' = 63.6% (22' = upper range of foreslope Clear-zone from Table 3.1)

Step 2.

(100%-70.0%) x 14' = 4.2' (14' = lower range of backslope CZ from 3.1)

(100%-63.6%) x 16' = 5.8' (16' = upper range of backslope CZ from 3.1)

The adjusted blackslope CZ is 4.2' - 5.8', or 4' - 6' (Answer B ).

I hope I helped.

 
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