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At-Grade Intersection Design

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Objectives

Define general intersection design objectives

Describe factors that influence curb return design

For a nice overview of intersection design, see http://www.fhwa.dot.gov/environment/flex/ch08.htm

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Intersections

Grade-separated without ramps Interchanges (grade separated with

ramps) At-grade

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Intersection – Design Controls• Primary controls:

• Functional class of roadways

• Topography and environment (manmade and natural)

• Secondary controls:• Design speed

• Design vehicles

• Traffic Characteristics (design volumes, level of service)

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Intersection Design Elements/Considerations

4 or fewer legs (within functional area) As close to 90 degrees as possible Approach (flat and straight as possible)

• Avoid > 6% on low speed (< 40 mph) and > 3% on high speed (≥ 50 mph)

Provide min. grades and max. vertical curve lengths

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Intersection Design Elements/Considerations

Continue grade throughout the intersection Make adjustments away from intersection Traffic lanes should be visible and obvious to

motorists Motorists should understand the path they are

supposed to take Horizontal and vertical alignment should facilitate

frequent braking

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Types of Intersection 4-leg T-intersection 5 or more legs Traffic circles

• Rotaries: large diameter > 300 ft, allows speeds > 30 mph with minimum horizontal deflection of the path of through traffic

• Neighborhood traffic circle: small diameter, for local streets, traffic calming

• Roundabout• Yield control at each approach

• Separation of conflicting movements

• Speed < 30 mph (typically)

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Basic Principles

Minimize severity of potential conflicts Provide for smooth flow of traffic Consider both vehicles and pedestrians

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Elements of Design

Design of alignment Design of channeling system Determination of minimum required

widths of turning roadways • Speeds > 15 mph

Intersection sight distance Determination of number of lanes

• Provision of turning lanes

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Alignment Horizontal

90° intersection of approaches

Skewed• Visibility

• Longer crossing times in some cases

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Profile (Vertical)

Should facilitate driver’s control of vehicle

Avoid significant changes in grade Typically ≤ 3% Continue major street grade through

intersection

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Curb Radius

Factors of influence:

• Design vehicle

• Intersection angle

• Approach width and parking

• Channelization

• Pedestrians

• Allowable speed reduction

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Curb Radius Green Book Minimums assume:

• Turning Speed LTE 10 mph

• Vehicle properly positioned beginning and end

• Vehicle doesn’t cross inner edge of pavement

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Design Vehicle

Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

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Radius Design

• Simple curve

•Low speed collector, local streets

• Simple circle with taper• 3-centered compound curve

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Minimize lane encroachment

R = 15 feet

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Simple Curve (passenger car template)

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Simple Curve with Taper (passenger car template)

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Compound curves (passenger car template)

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Curb Radius

General Guidance

•10 to 25 ft. local

•25 to 30 ft. collectors

•30 to 35 ft. unchannelized intersections with arterials

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Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

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High Speed Turns

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Channelization

Separates conflicting movements into definite paths of travel

Uses pavement markings or traffic islands Directs vehicle paths so no more than 2

paths cross at one point Controls merging, diverging, and crossing

angle of vehicles Decreases vehicle wander

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Channelization

Provides clear path for different movements

Gives priority to dominant movements Provides pedestrian refuge Provides storage area for turning vehicles Controls prohibited turns Restricts speed

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Types of Channelization

Raised islands• Urban

• Provides refuge for pedestrians

• <= 50 ft2 in urban areas

• <= 75 ft2 in rural areas Pavement markings

• Low pedestrian volume, low approach speeds

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Types of Channelization

Pavement edge• Rural – painted if high speed• Formed by diverging through and right turn

lanes

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Delineation With Pavement Marking

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Left & Right Turn Lane Warrants

Turning movement volumes

Accident experience

Capacity

Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

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Left & Right Turn Lane Design Number of likely queued vehicles

• Type of control

• Number of turning vehicles

• Length of vehicles

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Deceleration Lanes

Provides distance for turning vehicles to decelerate away without interfering with through traffic

Deceleration lane length depends on:• Speed

• number of queued vehicles

• vehicle length

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Median Openings …(see GB for Design specs)

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Intersection Design Considerations for

Pedestrians

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Provide median refuge to provide crossing in stages

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Provision of crosswalks

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Intersections on High Speed Rural Divided Expressways

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