Page 105 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

88 CHAPTER TWO

TABLE 2.6 Friction Factors for Design
Speeds from 20 to 70 mi/h (32 to 113 km/h);
Used in Developing Superelevation Tables
Design Side friction
speed, mi/h factor f
20 0.27
30 0.20
40 0.16
50 0.14
55 0.13
60 0.12
65 0.11
70 0.10
Source: Adapted from Ref. 1.

Highway design using U.S. Customary units defines horizontal curvature in terms
of degree of curve as well as radius. Under this definition, the degree of curve is
defined as the central angle of a 100-ft (30-m) arc using a fixed radius. This results in
the following equation relating R (radius, ft) to D (degree of curve, degrees):
5729.6
D (2.3)
R
Substituting in Eq. (2.2) gives the maximum degree of curvature for a given design
speed and maximum superelevation rate:
85,660(e f)
D (2.4)
max V 2
Before presenting the superelevation tables, one final consideration must be
addressed. Because for any curve, superelevation and side friction combine to offset
the effects of centrifugal force, the question arises how much superelevation should be
provided for curves flatter than the “maximum” allowed for a given design speed. The
following five methods have been used over the years (Ref. 1):

Method 1. Superelevation and side friction are directly proportional to the degree
of curve or the inverse of the radius.
Method 2. Side friction is used to offset centrifugal force in direct proportion to
the degree of curve, for curves up to the point where f max is required. For sharper
curves, f max remains constant and e is increased in direct proportion to the increasing
degree of curvature until e is reached.
max
Method 3. Superelevation is used to offset centrifugal force in direct proportion
to the degree of curve for curves up to the point where e max is required. For sharper
curves, e max remains constant and f is increased in direct proportion to the increasing
degree of curvature until f is reached.
max
Method 4. Method 4 is similar to 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 relationship with
the degree of curve (inverse of radius), with resulting values between those of
method 1 and method 3.