Superelevation tables

Hope everyone are studying well. I know every one must be busy at this time, but I cant help posting this question just to see if I can get a reply.

Well, the green book consists of a bunch of superelevation tables. I think I understand exhibit 3-15, which is straightforward since it has already given the maximum e and the corresponding values. But, exhibits 3-25 to 3-29 are confusing. They specified max e for each table and then given different radius for different superelevations less than the maximum value. This is very confusing to me.

For eg: for e max: 4% and e= say. 3.0, for a design speed of 30 the radius is 681 ft from exhibit 3-25.

I am expecting the radius to stay the same for the same set of values, i.e e=3% & v=30 when I use exhibit 3-26, but the radius is given as 1320. Why does this difference exist for the same set of e and V. What role does e max. Play in it ? The formula as given in CERM, equation 78.37(B) does not take into account e max. It just uses e values, which is 3% in the eg. Above.

Finally, how does 3-25 to 3-29 differ from 3-15.

I tried to be as clear as possible, sorry if I confused you. Just think that it is a stupid question if this confuses you too much and good luck to you on the exam! Do well, stay cool.

If it makes you feel any better winner, I have the same exact confusion. In fact, I also don't get Exhibits 3-17 - 3-23. I think we should be fine with Exh. 3-15 and equation (3-10). Good luck!

emax is used primarily for transition situations starting from a typical cross-slope (1.5%) to the max along the curve. If the curve is designed for emax = 4%, use the emax 4% table, not anything higher because the emax for the curve won't reach that level.

The reason for these values is that the driver won't slip outward from the curve while driving at any point along the transition from emin (1.5%) to emax.

Thanks for the clarification. I understand that it is a transition, but, the radius given are different for 1.5 and 4 from the emax =4% table. Does this mean that the radius is continuously changing, if so, dosen't it mean that the curve is not a simple curve? And why is R different for same e among different emax tables ?

Thanks

Don't think of it as the radius continually changing, but think of it as the minimum radius required for a car to travel along a curve with a final cross slope of e so that the car can stay on the road without slipping. The radius values other than e should be ignored when looking up the radius at e. For exam purposes, whatever happens before that point in the curve is irrelevant. Also, you're determining the MINIMUM radius, not the maximum. In the real world, there are many other factors in the design that usually results in exceeding the minimum radius.

R is different between emax tables because there are no true horizontal curves on highways in the real world, except for race tracks. Road designers have to take into account the minimum and maximum radii along a point of a curve with a specific cross-slope, OR design the cross-slope to meet the radius constraints knowing the maximum allowable cross-slope and design speed. This is where transition and spiral curves come into play.

For exam purposes, just use the emax table that matches the emax in the problem. For emax less than or equal to 4%, use the emax 4% table.

Well, despite sac's detailed explanation, I am still confused and I still don't get it......................... but it doesn't matter now - I passed anyway.