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At-Grade Intersection Design
CE 453 Lecture 32
For a nice overview of intersection design, see
http://www.fhwa.dot.gov/environment/flex/ch08.htm
Objectives
Basic intersection dimensions
Turning templates
High-speed intersections Other considerations
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Design Vehicle
Source:www\fhwa\Flexibility inHighway Design - Chapter 8- FHWA.htm
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Curb Radius
Factors of influence:
Design vehicleIntersection angleApproach width and parkingChannelizationPedestriansAllowable speed reduction
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Curb Radius
Green Book minimums assume:
Turning Speed 10 mph
Vehicle properly positioned beginningand endVehicle doesnt cross inner edge of
pavement
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Radius Designs
Simple curve Low speed collector, local streets
Simple circle with taper 3-centered compound curve
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Curb Radius
General Guidance 10 to 25 ft. local 25 to 30 ft. collectors
30 to 35 ft. unchannelized intersections witharterials
Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm
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Minimize lane encroachment
R = 15 feet
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Simple Curve (passenger cartemplate)
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Simple Curve with Taper(passenger car template)
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Compound curves (passengercar template)
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Higher Speed Turns
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Intersection Design
Considerationsfor 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 DividedExpressways
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High speed expresswayintersections
Iowa DOT is building many ruralexpressways
At-grade intersections can be verydangerous Dont use traffic signals as a safety
measure (in most cases) - leave as 2-way stop or replace with interchange
There are alternate measures
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Alternate measures Double yellow center line and stop bars in the median
maintain and replace frequently More and longer left and right turn lane
Offset! Paved shoulders near the intersections "T intersections Offset "T" intersections
much fewer conflict points Restricted median allow lefts in and rights in and out
prohibit lefts out and cross traffic
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Alternate measures Education about proper process via media (print or
electronic) stop, look left only and enter into the median stop in the median
look right and then cross or turn left onto theexpressway Advisory speed limit 10-mph below the posted speed
Signs on both sides (where fast aggressive driversoperate)
In low volumes, traffic may not slow down In peak hours one slows everyone slows
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US 65 Bondurant Example
Speed study done before and after
Some problems with the speedplates
Intense enforcement during thebefore and after period
Data show promise
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Another idea median sign west on Boone
on US-30
Initially solar poweredstrobe light above the signto draw attention
was distracting to driverson US- 30
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Change Blindness A psychological phenomenon Lots of roadside items = ease of estimating speed Clean roadside = not so easy Gray/white/blue sky lots of glare Older drivers
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Overhead intersectionbeacons?
Recent research inMinnesota ineffective confuse motorists
(drivers may think theflashing red beaconmeans 4-way stop)
now removing over headflashing intersectionbeacons in favor of redbeacon on the stop sign
Another Idea
Overhead lighting
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Circular Intersections
Rotaries: large diameter > 300 ft, allowsspeeds > 30 mph with minimum horizontaldeflection 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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DuPont Circle, Washington,
D.C. (Google Earth image)
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Urbandale
Image Google Earth
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A6 Motorway near Crooklands
Image Google Earth