Objective: Geometry of directional transition to ensure: Safety
Comfort Primary challenge Transition between two directions
Horizontal curves Fundamentals Circular curves Superelevation
Slide 4
Factor Affecting the Design of Horizontal Curves The Design
Speed of the road The Safe allowable Coefficient of Friction in the
lateral direction Maximum Allowable Rate of Superelevation, and
Permissible Centrifugal Ratio
Slide 5
Horizontal alignments with and without transition curves.
Slide 6
ELEMENTS OF A HORIZONTAL CURVE
Slide 7
Transition Curves Transition curves are used to connect
tangents to circular curves.
Slide 8
tangent to spiral point (TS), spiral to curve point (SC), curve
to spiral point (CS), spiral to tangent point (ST).
Slide 9
Length of Spiral Curve 1. According to rate of acceleration 2.
According to spiral geometry (A = spiral curve parameter) 3.
Minimum Length
Slide 10
F cp F cn WpWp WnWn FfFf FfFf FcFc W 1 ft e RvRv Forces acting
on a vehicle running on a horizontal Curve SUPERELEVATION
Slide 11
V (km/h) 50658095110 f 0.160.150.140.130.12 Case 1: Side
friction Only where f = coefficient of side friction V= design
speed (km/h) R= radius of horizontal curve (m) Case 2:
Superelevation Only where e = superelevation Speed is reduced to
avoid sliding inward during wet or icy seasons. Case 3:
Superelevation and Side friction For rural roads, e max = 0.10 -
0.12 For urban roads, e max = 0.08 - 0.10
Slide 12
Side Friction Factor from AASHTOs A Policy on Geometric Design
of Highways and Streets 2004
Slide 13
Minimum Radius Tables
Slide 14
WSDOT Design Side Friction Factors from the 2005 WSDOT Design
Manual, M 22-01 For Open Highways and Ramps
Slide 15
Design Superelevation Rates - AASHTO from AASHTOs A Policy on
Geometric Design of Highways and Streets 2004
Slide 16
Design Superelevation Rates - WSDOT from the 2005 WSDOT Design
Manual, M 22-01 e max = 8%
Slide 17
Superelevation Transition from the 2001 Caltrans Highway Design
Manual
Slide 18
Superelevation Transition from AASHTOs A Policy on Geometric
Design of Highways and Streets 2001
Slide 19
Spiral Curves WSDOT no longer uses spiral curves Involve
complex geometry Require more surveying Are somewhat empirical If
used, superelevation transition should occur entirely within
spiral
Slide 20
Spiral Curves No Spiral Spiral from AASHTOs A Policy on
Geometric Design of Highways and Streets 2001
Slide 21
No Spiral
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Slide 25
Stopping Sight Distance RvRv ss Obstruction MsMs SSD