Geometric Review and Design (GRAD) 3...Spiral Curves 3 16| Module 3 –Horizontal Alignment ODOT L&D...

Geometric Review and Design (GRAD)

Horizontal

Alignment

3

32 | Module 3 – Horizontal Alignment

ODOT L&D Vol. 1 – Section 200

Identify the Design Elements of Horizontal Alignments

Understand how to Design Horizontal Alignments to a Given Design Speed

Understand how to design appropriate superelevation design

Learning Objectives



Tangents

Circular Curves

Spiral (Transition) Curves

Superelevation

Elements of Horizontal Alignments



Sect 202

Maximum Deflection without

Horizontal Curve

Angle varies with the design speed of the roadway

Recommended Minimum Distance

between Consecutive Horizontal

Deflections

High speed / Low speed

Horizontal Alignments - Curves









Types of Curves

Simple Curves

Compound Curves

Reverse Curves

Spiral Curves




Key Elements of a Horizontal

Curve

Simple Curves



Degree vs. Radius

Degree of curve is the central angle subtended by a 100-foot arc for a given radius.

Example: Degree of curve for R = 1909 feet

Simple Curves



Degree vs. Radius



D = 3°

Simple Curves



Degree vs. Radius



D = 3°

Example: Radius of curve for D = 9.5°

Simple Curves



Degree vs. Radius



D = 3°

Example: Radius of curve for D = 9.5°

R = 603 feet

Simple Curves



2 horizontal curves in the same

direction

Utilize a common tangent

From AASHTO: It is preferable that the

ratio of the flatter radius to the

sharper radius not exceed 2:1 *

Should be used with caution!

Compound Curves

* A Policy on Geometric Design of Highways and Streets, 7th

Edition (2018) - Section 3.3.7.3



2 horizontal curves in opposite directions separated by a tangent with sufficient length to rotate superelevation from one curve to the next.

Superelevation - The cross slope of the pavement used to compensate for the effect of centrifugal force on horizontal curves

Reverse Curves



Combination of high speed and

sharp curvature leads to longer

transition paths, which can result in

shifts in lateral position and

sometimes actual encroachment

into adjacent lanes.

Should be used on new alignments

based on the maximum degree of

curve as shown in Figure 202-11.

Length of spiral should be equal or

greater than superelevation runoff

length for the curve.

Spiral Curves



Spiral Curves

Key Elements of a Spiral

Curve



Sect 202.4

Increasing the cross slope to

keep vehicles on the road

through the horizontal

curve

Implemented by raising the

pavement outer edge with

respect to the inner edge.

Superelevation



Recommended rates

Rural Highways (0.08 max) –

Figure 202-7

Urban Highways (high speed – 0.06

max) – Figure 202-8

Urban Ramps and Interchange

(low speed – 0.06 max) – Figure

202-10

Urban Highways (low speed – 0.04

max) – Figure 202-9; Figure 202-9a

Superelevation



Can be difficult to apply

recommended superelevation rates

on urban roadways because of:

Wide pavements

Adjacent development

Drainage conditions

Frequent access points

In these cases, may use reduced or

no superelevation although Crown

Removal is recommended min.

Superelevation – 0.04 Figure



Required whenever rate specified in Figures 202-7 through 202-10 is not provided.

Not required if a higher superelevation rate than what is specified in Figures 202-7 through 202-10 is provided as long as the rate doesn’t exceed the maximum superelevation rate for that given figure.

Superelevation – Design Exceptions



Maximum curvature (minimum

curve radius) that doesn’t

require superelevation based on

design speed and rural/urban

condition (Figure 202-3)

Can also use Figures 202-7, 202-

8, and 202-9 to determine these

values.

Maximum Curvature without Superelevation



Figures also show maximum

curvature for given design speed

(confirmed in Figure 202-2)

Maximum Curvature without Superelevation



Four methods:

1. Revolve pavement about the

centerline (most commonly used)

2. Revolve pavement about the inner

edge of traveled way

3. Revolve pavement about the outer

edge of traveled way

4. Revolve pavement having a straight

cross slope (no crown) about the

outside edge of traveled way (ramps

typically)

Superelevation Methods



Key Terms:

Normal Crown (Point A)

Adverse Crown Removed (Point B; also known as “half flat”)

Crown Removal (Point C)

PC/PT of curve (Point D) Superelevation rate is 50-70% design rate

Full Super (Point E) Design superelevation rate

Tangent Runout Normal Crown to Adverse Crown Removed

Superelevation Runoff Adverse Crown Removed to Full Super

Superelevation Transitions



With Spirals

Transition from Adverse Crown

Removed to Full Super shall occur

within the limits of the spiral.

Superelevation Position Spiral Length



With Spirals

Transition from Adverse Crown

Removed to Full Super shall occur

within the limits of the spiral.

Simple Curves – No Spirals

50%-70% of maximum

superelevation rate is outside the

curve limits; 2/3 of

superelevation rate at PC and PT

Prefer to have full superelevation

rate maintained for 1/3 of curve

length

Superelevation Position

2/3 (e(des) rate)



Determine width of pavement

that is being rotated






Identify Adjustment Factor (bw)

“Credit” for rotating more than

one lane








one lane

Identify Equivalent Slope Rate (G)

For the given Design Speed








one lane



Calculate Superelevation Runoff

(Lr)








one lane



Calculate Superelevation Runoff

(Lr)

Calculate Tangent Runout (Lt)




Example - Determine Superelevation

Degree of Curve = 3.0°

Design Speed = 60 mph

Urban Location

Superelevation






Urban Location

Radius of Curve = 1,910 feet

Superelevation






Urban Location


e(des) = 0.055 ft/ft

Superelevation






Urban Location

Superelevation






Urban Location

Degree of Curve = 3°42’

Superelevation






Urban Location

Degree of Curve = 3°42’


Superelevation



Example – determine

superelevation rotation

placement


Undivided Highway

Rotate 2-12’ lanes

PC Station = 100+73.35







placement


Undivided Highway




Superelevation Transitions or





placement


Undivided Highway




Superelevation Transitions or





placement


Undivided Highway









placement


Undivided Highway









placement


Undivided Highway









placement


Undivided Highway









placement


Undivided Highway









placement


Undivided Highway







Each curve has its own superelevation runoff value

New Alignments

PT and PC should be separated enough

for a smooth transition at a rate not

to exceed “G” value in Figure 202-4.

Distance to be not less than 50% nor

greater than 70% of Lr1 + Lr2. 2/3

(67%) is preferred.

Existing Alignments

Conform as closely as possible to New

Alignments criteria.

Superelevation - Reverse Curves





placement


Undivided Highway







Superelevation Exaggerated Profiles




Y = (enc) X

(rotating pavement width)




Y = (enc) X


Y = (FS cross slope) X


Y = (-FS cross slope) X





Y = (enc) X









Y = (enc) X






PC = 2/3 x Superelevation Rate)




Y = (enc) X






PC = 2/3 x Superelevation Rate)L = [(ePC/eDES) x Lr]




Y = (enc) X






PC = 2/3 x Superelevation Rate)L = [(ePC/eDES) x Lr]

L = Lt L = Lt






Information reviewers

should look for:

Superelevation Rate

Transition Length

Transition Rate

Super Rate at PC

Super Rate at PT

Length of curve/time at full superelevation

Superelevation Review



For horizontal alignment

design elements that

don’t meet standards

Chevrons

Rumble Strips

Curve Ahead Signage

Curve Widening

Place the wider shoulder on the left (instead of right) side

Safety Countermeasures



Key Terms

Horizontal Alignments

Tangents

Curves

Spirals

Horizontal Curves

Simple Curves

Compound Curves

Reverse Curves

Spiral Curves

Tangent Runout (NC to ACremoved)

Superelevation Runoff (ACremoved to FS)

Module Review

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Geometric Review and Design (GRAD) 3...Spiral Curves 3 16| Module 3 –Horizontal Alignment ODOT L&D...

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