At-Grade Intersection Design

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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
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Design Vehicle
Source www\fhwa\Flexibility in Highway Design -
Chapter 8 - FHWA.htm
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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 doesnt cross inner edge of pavement

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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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Curb Radius

• General Guidance
• 10 to 25 ft. local
• 25 to 30 ft. collectors
• 30 to 35 ft. unchannelized intersections with
  arterials

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 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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High Speed Turns
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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 - Tom Welch, State Traffic Safety
Engineer
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High speed expressway intersections

• We are building many rural expressways
• At-grade intersections can be very dangerous
• 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

• install double yellow center line and stop bars
  in the median
• maintain and replace frequently
• When resurfacing, look for opportunities to
  install more and longer left and right turn lanes
• Paving shoulders near the intersections
• Build "T intersections or even offset "T"
  intersections
• much fewer conflict points
• Use a restricted median
• allow lefts in and rights in and out
• prohibit lefts out and cross traffic

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Alternate measures

• ask local newspaper to publish an article which
  explains the proper way to cross and enter a
  divided roadway.
• stop, look left only and enter into the median
• stop in the median
• look right and then cross or turn left onto the
  expressway
• Wisconsin pioneered advisory speed limit 10mph
  below the posted speed
• Signs need to be on both sides to work (where
  fast aggressive drivers operate)
• In low volumes, traffic may not slow down
• In peak hours (most Important because most
  turns), almost always one person in the platoon
  slows down - whole platoon slows

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US 65 Bondurant Example

• Speed study done before and after
• Some problems with the speed plates
• Intense enforcement during the before and after
  period
• Data shows promise

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Another idea

• median sign west on Boone on US-30
• Initially solar powered strobe light above the
  sign to draw attention
• was distracting to drivers on US- 30

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Change Blindness

• a psychological phenomenon
• In the past, motorists would pull up to a stop
  sign at a rural two lane highway and when they
  looked down the highway there would all kinds of
  buildings, fence posts tree etc along the narrow
  R/W and roadway
• They could easily note the change in position of
  approaching vehicles and better judge the
  approaching vehicular closing speed.
• Nowadays on our expressways, we cleanout the wide
  R/W, there are fewer buildings and trees along
  the R/W etc.
• So, now there is nothing left to help the driver
  judge the change in position of the approaching
  vehicles on the expressway.
• Imagine now a 75 year old driver with restricted
  eye sight, trying to judge the speed of a light
  colored vehicle up against the gray/ white
  horizon.
• Tough to judge approaching speeds

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Overhead intersection beacons?

• Recent research in Minnesota
• ineffective
• confuse motorists (drivers may think the flashing
  red beacon means 4 way stop)
• now removing over head flashing intersection
  beacons in favor of red beacon on the stop sign

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Traffic circles

• Rotaries large diameter gt 300 ft, allows speeds
  gt 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 lt 30 mph (typically)

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