City of Frisco Engineering Standards

Engineering Standards
TABLE 2.2: Minimum Horizontal Centerline Radius
Design Speed,
V (MPH)
Friction Factor,
F
Superelevation,
e (ft/ft)
Radius, R (ft)
(Rounded for Design)
25 0.23 -0.02 250 (1)
30 0.20 -0.02 350
35 0.18 -0.02 525
40 0.16 -0.02 775
45 0.15 -0.02 1100
50 0.14 -0.02 1400
(1) May be reduced to a radius of two hundred feet (200’) at mid-block locations provided that it is shown that the
general public safety is not compromised (e.g., stopping sight distance). A curve with a radius less than two
hundred fifty feet (250’) must be a minimum of three hundred feet (300’) from a street or alley intersection.
D. Minimum Vertical Alignment
1. Vertical curves are utilized in roadway design to affect gradual change between tangent
grades and will result in a design which is safe, comfortable in operation, pleasing in
appearance and adequate for drainage. Vertical curve alignment shall also provide Stopping
Sight Distance (SSD) in all cases. SSD is a function of design speed, perception-reaction
time, and deceleration rate. The perception-reaction time is assumed to be 2.5 seconds and is
in accordance with the guidelines in AASHTO’s 2011 edition of A Policy on Geometric
Design of Highways and Streets. The equation for SSD appears below:
V
SSD = 1 .47Vt
+ 1.075
a
SSD = stopping Sight Distance (ft)
t = brake reaction time (2.5 sec.);
V = vehicle design speed (MPH); and
a = deceleration rate, (11.2 ft/s 2 )
2. To determine the minimum acceptable length of Crest and Sag curves shown in Tables 2.3
and 2.4, it is assumed that approach grades are between -3% and 3% in the SSD calculation.
The SSD for grades steeper than -3 % or 3% shall be in accordance with the guidelines in
AASHTO’s current A Policy on Geometric Design of Highways and Streets. Tables 2.3 and
2.4 also show values of K. K is defined as the rate of vertical curvature and is equivalent to
the horizontal distance in feet required to make a one percent (1%) change in grade. The
values of A are equivalent to the algebraic difference in grade between the two grades that are
being joined together by the vertical curve.
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Section 2 – Thoroughfare Design Requirements May 2012 Page 2-9