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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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