Page 118 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

HIGHWAY LOCATION, DESIGN, AND TRAFFIC 101

a different maximum superelevation rate. Table 2.7 shows values for a maximum rate
of 0.04; Table 2.8, for 0.06; Table 2.9, for 0.08; Table 2.10, for 0.10; and Table 2.11,
for 0.12. Method 5 was used to calculate the minimum radius for each superelevation
rate less than the maximum rate in each design speed column in the tables.
The superelevation rates on low-speed urban streets are set using method 2 described
above, in which side friction is used to offset the effect of centrifugal force up to the
maximum friction value allowed for the design speed. Superelevation is then introduced
for sharper curves. The design data in Table 2.12, based on method 2 and a maximum
superelevation rate of 0.04, can be used for low-speed urban streets and temporary
roads. The design data in Table 2.13 can be used for a wider range of design speeds and
superelevation rates.
In attempting to apply the recommended superelevation rates for low-speed urban
roadways, various factors may combine to make these rates impractical to obtain.
These factors include wide pavements, adjacent development, drainage conditions,
and frequent access points. In such cases, curves may be designed with reduced or no
superelevation, although crown removal is the recommended minimum.

Effect of Grades on Superelevation. On long and fairly steep grades, drivers tend to
travel somewhat slower in the upgrade direction and somewhat faster in the downgrade
direction than on level roadways. In the case of divided highways, where each pavement
can be superelevated independently, or on one-way roadways such as ramps, this ten-
dency should be recognized to see whether some adjustment in the superelevation rate
would be desirable and/or feasible. On grades of 4 percent or greater with a length of
1000 ft (305 m) or more and a superelevation rate of 0.06 or more, the designer may
adjust the superelevation rate by assuming a design speed 5 mi/h (8 km/h) less in the
upgrade direction and 5 mi/h (8 km/h) greater in the downgrade direction, provided
that the assumed design speed is not less than the legal speed. On two-lane, two-way
roadways and on other multilane undivided highways, such adjustments are less feasible,
and should be disregarded.

Superelevation Methods. There are three basic methods for developing superelevation
on a crowned pavement leading into and coming out of a horizontal curve. Figure 2.9
shows each method. In the most commonly used method, case I, the pavement edges are
revolved about the centerline. Thus, the inner edge of the pavement is depressed by half
of the superelevation and the outer edge raised by the same amount. Case II shows the
pavement revolved about the inner or lower edge of pavement, and case III shows the
pavement revolved about the outer or higher edge of pavement. Case II can be used
where off-road drainage is a problem and lowering the inner pavement edge cannot be
accommodated. The superelevation on divided roadways is achieved by revolving the
pavements about the median pavement edge. In this way, the outside (high side) roadway
uses case II, while the inside (low side) roadway uses case III. This helps control the
amount of “distortion” in grading the median area.
Superelevation Transition. The length of highway needed to change from a normal
crowned section to a fully superelevated section is referred to as the superelevation
transition. This length is shown as X in Fig. 2.9, which also shows the various other
elements described below. The superelevation transition is divided into two parts: the
tangent runout, and the superelevation runoff.
The tangent runout (T in Fig. 2.9) is the length required to remove the adverse
pavement cross slope. As is shown for case I of Fig. 2.9, this is the length required to
raise the outside edge of pavement from a normal cross slope to a half-flat section.
The superelevation runoff (L in Fig. 2.9) is the length required to raise the outside