Page 104 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

HIGHWAY LOCATION, DESIGN, AND TRAFFIC 87

When centerline deflections exceed the values in Table 2.5, it is necessary to introduce
a horizontal curve to assist the driver. Curves are usually accompanied by supereleva-
tion, which is a banking of the roadway to help counteract the effect of centrifugal
force on the vehicle as it moves through the curve. In addition to superelevation, cen-
trifugal force is also offset by the side friction developed between the tires of the vehicle
and the pavement surface. The relationship of the two factors when considering curvature
for a particular design speed is expressed by the following equation:
V 2
U.S. units: e f (2.1a)
15R
V 2
SI units: e f (2.1b)
127R

where e superelevation rate, ft per ft (m per m) of pavement width
f side friction factor
V design speed, mi/h (km/h)
R radius of curve, ft (m)

In developing superelevation guidelines for use in designing roadways, it is necessary
to establish practical limits for both superelevation and side friction factors. Several
factors affect the selection of a maximum superelevation rate for a given highway.
Climate must be considered. Regions subject to snow and ice should not be superelevated
too sharply, because the presence of these adverse conditions causes motorists to drive
slower, and side friction is greatly reduced. Consequently, vehicles tend to slide to the low
side of the roadway. Terrain conditions are another factor. Flat areas tend to have rela-
tively flat grades, and such conditions have little effect on superelevation and side friction
factors. However, mountainous regions have steeper grades, which combine with super-
elevation rates to produce steeper cross slopes on the pavement than may be apparent
to the designer. Rural and urban areas require different maximum superelevation rates,
because urban areas are more frequently subjected to congestion and slower-moving
traffic. Vehicles operating at significantly less than design speeds necessitate a flatter
maximum rate. Given the above considerations, a range of maximum values has been
adopted for use in design. A maximum rate of 0.12 or 0.10 may be used in flat areas
not subject to ice or snow. Rural areas where these conditions exist usually have a
maximum rate of 0.08. A maximum rate of 0.06 is recommended for urban high-speed
roadways, 50 mi/h (80 km/h) or greater, while 0.04 is used on low-speed urban roadways
and temporary roads.
Various factors affect the side friction factors used in design. Among these are
pavement texture, weather conditions, and tire condition. The upper limit of the side
friction factor is when the tires begin to skid. Highway curves must be designed to
avoid skidding conditions with a margin of safety. Side friction factors also vary with
design speed. Higher speeds tend to have lower side friction factors. The result of various
studies leads to the values listed in Table 2.6, which shows the side friction factors by
design speed generally used in developing superelevation tables (Ref. 1).
Taking into account the above limits on superelevation rates and side friction factors, and
rewriting Eq. (2.1), it follows that for a given design speed and maximum superelevation
rate, there exists a minimum radius of curvature that should be allowed for design purposes:
V 2
R (2.2)
min 15(e f)
To allow a lesser radius for the design speed would require the superelevation rate or
the friction factor to be increased beyond the recommended limit.