Alberta RoadsideDesign Guide

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Module 3Clear Zone Concept

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Roadside Design Process

Data Requirements

Design Speed

AADT

Foreslope and Backslope Information

Horizontal Curves

Features within the ROW

• Bridges

• Culverts

Determine Desirable Clear Zone

Identify Hazards

Examples:Sideslope steeper than 2 to 1

Trees with diameter of 100mm or more

Fixed objects extending above the ground surface by more than 100mm such as bridge rail ends, bridge abutments, piers, or retaining wall ends

Non-breakaway signs and light pole supports, and utility poles

Vertical drops greater than 300mm

Bodies of water with depth of 1m or more

Data Requirements

Remove Hazard

Redesign Hazard

Relocate Hazard

Reduce Severity Hazard

Shield Hazard

Delineate Hazard

Figure H3.1

Page H3-1

The Clear Zone Concept

• The clear zone is defined as the border area starting at the edge of the travel lane that should be clear of hazards and available for use by errant vehicles

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Glossary H1.6

Page H1-8

The Clear Zone 

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ObstacleTravelled Way

Shoulder

RecoverableSlope

Non-recoverable Slope

Runout Area

Clear Zone Distance

Recovery Zone

The Clear Zone

• A traversable area that starts at the edge of the traffic lane, includes the shoulder, and extends laterally a sufficient distance to allow a driver to stop or return to the road before encountering a hazard or overturning.

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The Recovery Zone

• Total unobstructed traversable area• Measured from edge of closest travel lane

• May have– Recoverable slopes– Non‐recoverable slopes– Clear runout area

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The Clear Zone

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EncroachmentProbabilities :Foundation forthe Clear Zone

20%

10 m

Probability

Encroachment

Recoverable Slopes

• Recoverable slopes are those on which a driver may to a greater or lesser extent, retain or regain control of an errant vehicle.  

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Slopes 4:1 or flatter, are generally considered recoverable.

Non‐recoverable Slopes

• A non‐recoverable slope may be traversable, but a vehicle will generally continue to the bottom with no chance of recovery. 

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Embankment slopes between 3:1 and 4:1 maybe traversable, but non-recoverable.

Run‐Out Area

• A clear runout area is located at the toe of a non‐recoverable slope and is available for safe use by an errant vehicle

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Roadside Recovery Zone

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Note: If the clear zone distance ends on a non-recoverable slope a clear runout area is required.

Clear Zone

• The clear zone reflects the influence of:

• design speed• traffic volume• The presence of cut or fill slopes• Horizontal curve adjustments

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Embankment slope definitions

• Recoverable slope: 4:1 or  flatter– Drivers may retain or regain control– No need for recovery area at Toe of Slope

• Non‐recoverable:  between 3:1 & 4:1– Traversable – Recovery area required at Toe of Slope

• Critical slopes: steeper than 3:1– Overturning likely– Constitutes an obstacle

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Example of a Parallel Embankment Slope Design

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Clear Zone Distances(in metres from edge of driving lane)

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Table H3.1Page H3-4

Desirable Clear Zone (DCZ)

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Width of Adjacent roadside border area for use by an errant vehicle.

Note:

Clear Zone = Desirable Clear Zone

Clear Zone Adjustment Equation

Where:

DCZ = Desirable Clear Zone

DTCZ = Clear Zone for a tangent highway

KCZ = Curve Correction Factor

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DCZ = (DTCZ) x (Kcz)

Curve Modification Factor (KCZ)

Radius 60 70 80 90 100 >110>1100

1.0 1.0 1.0 1.01.0 1.0

1100 1.1 1.1

900

1.1

1.11.1

1.21.2

1.2

700

1.2

1.3

600

1.2

1.3 1.4500

1.3450

1.21.3

1.41.5

400

350 1.41.5

3001.3 1.4 1.5

2501.3

200 1.4 1.5

150 1.4 1.5

100 15

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Design Speed (km/h)

Notes:

(1) Clear Zone correction factor applied to outside of curves only

(2) Curves flatter than 1100m do not require adjusted Clear Zone

Table H3.2Page H3-5

Notes on KCZ

• Applies to outside of curves • For R > 1100m KCZ not required

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Desirable Clear Zone

DCZ should not be considered the maximum clear distance

Mitigation should be considered for:• Cliffs• Water• Bridge piers• Terrain that channels errant vehicles to a hazard

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Hazards in Clear Zones

• Sideslopes• Roadside obstacles• Permanent body of water• Barriers (low severity, SI   4.0)

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Sideslopes and Backslopes

• Sideslope Ratio > 3:1 are a hazard• High embankments are hazards• Steep backslopes are hazards

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Barrier Warrants for fill slopes where AADT<400

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FigureH3.4Page H3-8

Barrier Warrants For Fill Slopes Where AADT>400

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FigureH3.5Page H3-9

Roadside Obstacles

• Non‐traversable (eg. drainage device)• Fixed object

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Hazards for Mitigation• Wood poles > (100mm x 100mm)• Trees Diameter >100mm• Fixed objects extending > 100mm

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Hazards for Mitigation

• Non‐breakaway sign supports• Non‐breakaway utility poles• Shoulder drop‐off > 300mm

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Mailboxes are a Hazard• Wood post > 100mm• Steel post > 50mm

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Drainage Structures are Hazards

• With a diameter > 600mm• Must be mitered• Have piperunners*• Headwalls must be shielded

*Note: Piperunners are not mentioned in  the ARDG, however are considered  good practice.

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Mitigation Strategies

• Consider at grading design stage

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Underpasses• Design bulletin #22/2004 barrier is normally required if pier is located in the Clear Zone.(Ref: Figure H7.1 in ARDG)

• A suitable minimum offset for an unprotected 2:1 backslope for any given speed is ½ the Clear Zone

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Clear Recovery Zone

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Edge of Clear Zone

Lesson Learned From Clear Zones in Canada

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Ditch blocks, when required may make the clear zone non-traversable

Boulders in Clear Zone

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Boulders in Clear Zone

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Shielded Poles

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• Note distance from edge of highway

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Unshielded Poles in Clear Zone

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Curb Usage

• Curbs may be required due to R/W restrictions, drainage, access control or other curb functions.

• However curbs are discouraged for high speed roadways (i.e. ≥ 70 km/h).

• See RDG Table H4.1

Operating Speed

Table H4.1

AlbertaRoadsideDesignGuide

Revised July 2009

Excerpt from RDG Table H4.1

Installation of curb/barrier combinationnot recommended.

Operating Speed> 100 km/h

Revised July 2009

Excerpt from RDG Table H4.1

OperatingSpeed

>85 - 100Only Mountable Curb is permitted with barrier

Revised July 2009

Excerpt from RDG Table H4.1

OperatingSpeed

>60 - 70

Mountable and Semi-mountable maybe used depending on offset.

Revised July 2009