Using Green Book to find minimum radius of horiz. curve

[maximus808]

Please someone correct me if I'm wrong but are there only two different situations where you would use two different set of tables for finding the min. radius given a max superelevation.

For example, I had a question in the 6 min. solutions that referred to Exhibit 3-16 because the road was considered urban low speed vs. any other scenario I would use Exhibit 3-27 tables given a speed and max. superelevation. I didn't realize there two different set of tables for different area types with different speeds. Can someone provide some insight. Thanks.

 

[sac_engineer]

Please someone correct me if I'm wrong but are there only two different situations where you would use two different set of tables for finding the min. radius given a max superelevation.
For example, I had a question in the 6 min. solutions that referred to Exhibit 3-16 because the road was considered urban low speed vs. any other scenario I would use Exhibit 3-27 tables given a speed and max. superelevation. I didn't realize there two different set of tables for different area types with different speeds. Can someone provide some insight. Thanks.

You're correct that the superelevation rates differ depending on the situation. An exam problem would most likely provide the e-max value rather than you having to assume it. Also, I would stick to Exhibit 3-15 for the radius values.

Exhibits 3-15 and 3-27 to 3-29 are essentially the same tables, but the latter set has smaller increments of the superelevation value for more detailed designs.

When all else fails, use the generic formula:

R(min) = S^2 / [15 x (emax + f)]; S = design speed (mph), emax = max. superelev (decimal), f = side friction factor (default = 0.18)

Good luck!