Does anyone know where the CERM's 2/3-1/3 Rule for superelevation comes from?

[ptatohed]

I am helping my friend study and we came across this.

In the CERM (pg 79-9 in the 13th ed) they discuss a 2/3 - 1/3 rule for superelevation. Meaning, of the total superelevation transition length (made up of the tangent runout, Lt, and the superelevation runnoff, Lr), the superelevation runnoff portion stradles the curve's BC, with 2/3 of the total Lr length before the BC and 1/3 after the BC (2/3 on the tangent, 1/3 on the curve).

I can't find this rule in any of the NCEES approved references (most particularly, the GDHS).

Table 3-16 of the 2011 6th ed GDHS lists the portion of Lr that comes before the BC. Depending on the design speed and number of lanes rotated, it ranges from 0.70 to 0.90. Not the 0.66 that the CERM's 2/3 - 1/3 rule would result in.

Am I missing something? Does anyone know the origin of the 2/3 - 1/3? Is it valid? Or is Table 3-16 the correct method to use?

Thanks.

 

[Predgw]

Page 3-67 has additional guidance if the 2/3 rule does not provide enough cross slop on wider pavement surfaces or on lower design speeds.

 

[ptatohed]

Page 3-67 has additional guidance if the 2/3 rule does not provide enough cross slop on wider pavement surfaces or on lower design speeds.

I totally missed that "most agencies use 0.67" staring me in the face, first line of pg 3-67. Thanks!