1. Horizontal Alignment
See: (Chapter 5 from FHWA’s Flexibility in Highway Design)

2. Horizontal Alignment
Design based on appropriate relationship between design speed and curvature and their interaction with side friction and superelevation
Along circular path, inertia causes the vehicle to attempt to continue in a straight line
Superelevation and friction between tire and roadway provides a force to offset the vehicle’s inertia; this force is directed toward the center of curvature (often called centrifugal or centripetal force)

3. Horizontal Alignment Tangents Curves Transitions
Curves require superelevation (next lecture)
Reason for super: banking of curve, retard sliding, allow more uniform speed, also allow use of smaller radii curves (less land)

4. Radius Calculation Rmin = ___V2______ 15(e + f) Where:
V = velocity (mph)
e = superelevation
f = friction (15 = gravity and unit conversion)

5. Radius Calculation Rmin related to max. f and max. e allowed
Rmin use max e and max f (defined by AASHTO, DOT, and graphed in Green Book) and design speed
f is a function of speed, roadway surface, weather condition, tire condition, and based on comfort – drivers brake, make sudden lane changes and changes within a lane when acceleration around a curve becomes “uncomfortable”
AASHTO: 20 mph with new tires and wet pavement to 60 mph
f decreases as speed increases (less tire/pavement contact)

6. Max e Controlled by 4 factors:
Climate conditions (amount of ice and snow)
Terrain (flat, rolling, mountainous)
Frequency of slow moving vehicles who might be influenced by high superelevation rates
Highest in common use = 10%, 12% with no ice and snow on low volume gravel-surfaced roads
8% is logical maximum to minimized slipping by stopped vehicles

7. Source: A Policy on Geometric Design of Highways and Streets (The Green Book). Washington, DC. American Association of State Highway and Transportation Officials, th Ed.

10. Radius Calculation (Example)
Design radius example: assume a maximum e of 8% and design speed of 60 mph, what is the minimum radius?
fmax = 0.12 (from Green Book)
Rmin = _____602________________
15( )
Rmin = 1200 feet

11. Radius Calculation (Example)
For emax = 4%?
Rmin = _____602________________
15( )
Rmin = 1,500 feet

12. Curve Types Simple curves with spirals
Broken Back – two curves same direction (avoid)
Compound curves: multiple curves connected directly together (use with caution) go from large radii to smaller radii and have R(large) < 1.5 R(small)
Reverse curves – two curves, opposite direction (require separation typically for superelevation attainment)

13. Important Components of Simple Circular Curve
See: ftp:// /design/dmanual/02a-01.pdf
1.     See handout
2.     PC, PI, PT, E, M, and 
3.     L = 2()R()/360
4.     T = R tan (/2)
Source: Iowa DOT Design Manual

14. Sight Distance for Horizontal Curves
Location of object along chord length that blocks line of sight around the curve
m = R(1 – cos [28.65 S]) R
Where:
m = line of sight
S = stopping sight distance
R = radius

15. Sight Distance Example
A horizontal curve with R = 800 ft is part of a 2-lane highway with a posted speed limit of 35 mph. What is the minimum distance that a large billboard can be placed from the centerline of the inside lane of the curve without reducing required SSD? Assume p/r =2.5 and a = 11.2 ft/sec2
SSD = 1.47vt + _________v2____
30(__a___  G)
32.2

16. Sight Distance Example
SSD = 1.47(35 mph)(2.5 sec) +
_____(35 mph)2____ = 246 feet
30(__11.2___  0)
32.2

17. Sight Distance Example
m = R(1 – cos [28.65 S]) R
m = 800 (1 – cos [28.65 {246}]) = 9.43 feet
800
(in radians not degrees)

18. Horizontal Curve Example
Deflection angle of a 4º curve is 55º25’, PC at station Find length of curve,T, and station of PT.
D = 4º
 = 55º25’ = º
D = _ _ R = _ _ = 1,432.4 ft R

19. Horizontal Curve Example
D = 4º
 = º
R = 1,432.4 ft
L = 2R = 2(1,432.4 ft)(55.417º) = ft

20. Horizontal Curve Example
D = 4º
 = º
R = 1,432.4 ft
L = ft
T = R tan  = 1,432.4 ft tan (55.417) = ft

21. Horizontal Curve Example
Stationing goes around horizontal curve.
What is station of PT?
PC =
L = ft =
Station at PT = ( ) + ( ) =

22. Suggested Steps on Horizontal Design
Select tangents, PIs, and general curves make sure you meet minimum radii
Select specific curve radii/spiral and calculate important points (see lab) using formula or table (those needed for design, plans, and lab requirements)
Station alignment (as curves are encountered)
Determine super and runoff for curves and put in table (see next lecture for def.)
Add information to plans