4.7.7 Superelevation Transition Placement

The transition with respect to the termini of a simple (circular) curve should be placed to minimize lateral acceleration and the vehicle's lateral motion. The recommended allocation of superelevation transition on the tangent, preceding or following a curve, is provided on
Table 4-10
. For superelevation on bridge structures, it is preferred to begin/end superelevation at the bridge bent line. When spiral curves are present on an existing facility and alignment modifications aren’t practical, refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for transition distribution.
Table 4-10: Portion of Superelevation Transition Located on the Tangent
1
Design Speed (mph)
No. of Lanes Rotated
1.0
1.5
2.0 - 2.5
3.0 - 3.5
15 - 45
0.80
0.85
0.90
0.90
50 - 80
0.70
0.75
0.80
0.85
Notes:
  1. These values are recommendations based on prevailing research. A value between 0.7 and 0.9 for all speeds and rotated widths is considered acceptable. Refer to AASHTO’s A Policy on Geometric Design of Highways and Streets for additional information.
Care must be exercised in designing the length and location of the superelevation transition. Pavement surfaces should be modeled to ensure proper drainage, especially near the high or low portions of Type I or III vertical curves (see
Figure 4-5
for curve types).
A plot of roadway contours may assist with the verification of grades and identification of drainage problems in areas of superelevation transition. Preferably,
a minimum profile grade line (PGL) of 0.5 percent and minimum edge-of-pavement (EOP) profile grade of 0.2 percent (0.5 percent for curbed roadways) should be maintained throughout the
superelevation transition section
.
 At a minimum, either criterion should be met.
On existing alignments, whenever reverse curves are closely spaced and superelevation transition lengths overlap, transition lengths (L
CT
) should be adjusted to ensure that roadway cross slopes are in the proper direction for each horizontal curve. For proposed construction of new facilities, the tangent section between reverse curves should be of sufficient length such that minimum transition lengths for each transition do not overlap.