4C. Superelevation

Question 1

What is the maximum superelevation rate?

Answer 1

AASHTO Chapter 3: 3-32

Some highway agencies have adopted a maximum superelevation rate of 8 percent. Such agencies believe that 8 percent represents a logical maximum superelevation rate, regardless of snow or ice conditions.
A superelevation rate of 12 percent appears to represent a practical maximum value where snow and ice do not exist.
The highest superelevation rate for highways in common use is 10 percent.
Superelevation rates above 8 percent are only used in areas without snow and ice.

Question 2

What is the formula to calculate minimum radius?

Answer 2

AASHTO: Chapter 3: Equation 3-8

Rmin = V^2 / (15(0.01emax + fmax))

emax = max cross slope
fmax = maximum slide factor

Question 3

Where can a table for finding the minimum radius be found

Answer 3

AASHTO Green Book: Chapter 3: Table 3-7

The table relates design speed, maximum e and f variables, and radius.
Useful if given emax and fmax or radius and design speed.

Question 4

What are the five methods for sustaining lateral acceleration on curves by use of e or f or both?

Answer 4

AASHTO: Chapter 3:

Method 1—Superelevation and side friction are directly proportional to the inverse of the radius (i.e., a straight-line relation exists between 1/R = 0 and 1/R = 1/Rmin).

Method 2—Side friction is such that a vehicle traveling at design speed has all lateral acceleration sustained by side friction on curves up to those designed for fmax. For sharper curves, f remains equal to fmax and superelevation is then used to sustain lateral acceleration until e reaches emax. In this method, first f and then e are increased in inverse proportion to the radius of curvature.

Method 3—Superelevation is such that a vehicle traveling at the design speed has all lateral acceleration sustained by superelevation on curves up to those designed for emax. For sharper curves, e remains at emax and side friction is then used to sustain lateral acceleration until f reaches fmax. In this method, first e and then f are increased in inverse proportion to the radius of curvature.

Method 4—This method is the same as Method 3, except that it is based on average running speed instead of design speed.

Method 5—Superelevation and side friction are in a curvilinear relation with the inverse of the radius of the curve, with values between those of Methods 1 and 3.

Question 4

What formula can be used to calculate the e and f for superelevation?

Answer 4

AASHTO Chapter 3: Formula 3-9

0.01e + f = V^2 / (15*R)

where:

V = VD = design speed, mph
e = emax = maximum superelevation, percent
f = fmax = maximum allowable side friction
factor
R = Rmin = minimum radius, ft
then:

Question 5

Where can a table be found for the minimum radii for design superelevation

Answer 5

AASHTO Chapter 3: Table 3-8

Question 6

What formula should be used for calculating the minimum length of runnoff for a superelevated curve

Answer 6

AASHTO Chapter 3: Table 3-23

Lr = (((wn1)ed) / Δ ) (bw)

where:
Lr = minimum length of superelevation runoff, ft
w = width of one traffic lane, ft (typically 12 ft)
n1 = number of lanes rotated
ed = design superelevation rate, percent
bw = adjustment factor for number of lanes rotated (table 3-15)
Δ = maximum relative gradient, percent

Table 3-16 has precalculated data

Question 7

What are the 4 methods of attaining superelevation

Answer 7

AASHTO Chapter 3: Figure 3-8

(1) revolving a traveled way with normal cross slopes about the centerline profile,
(2) revolving a traveled way with normal cross slopes about the inside-edge profile,
(3) revolving a traveled way with normal cross slopes about the outside-edge profile,
(4) revolving a straight cross slope traveled way about the outside-edge profile.