Intersection Design

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Intersection Design

• Spring 2007

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Intersections
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Intersections
Depends on vehicle speed, driver alertness, and
driver familiarity with the location
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Vertical Alignment
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Vertical Alignment
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Turning Roadways

• Minimum Edge-of-Traveled-Way (METW)
• The corner radii should be based on the minimum
  turning path of the selected design vehicles
• Design paths of design vehicles illustrated in
  Exhibits 2-3 to 2-23
• METW shown in Exhibits 9-19 and 9-20
• METW dependent upon angle at which the highways
  meet
• Three types of design
• 1) simple curves
• 2) simple curves with taper
• 3) compound curves

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Turning Roadways
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Turning Roadways
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Turning Roadways
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Turning Roadways
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Turning Roadways
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Turning Roadways
Source NC DOT
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Turning Roadways
Example
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Turning Roadways
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Turning Roadways
Example
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Turning Roadways

• Summary for METW
• P design vehicle local road intersections with
  major roads where turns are made only
  occasionally
• SU truck design vehicle recommended minimum for
  rural highways
• Semitrailer combination used where truck
  combinations will turn repeatedly three-centered
  compound curves may be preferred may be
  desirable to build corner triangular island (to
  minimize asphalt overlay)

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Islands

• Islands
• Area between traffic lanes used for controlling
  vehicle movements
• Purposes
• Separate conflicts
• Control angle of conflict
• Reduce excessive pavement area
• Regulate traffic
• Favor a predominant turning movement
• Protect pedestrians
• Provide storage for vehicles
• Allow the use of traffic control devices
• Drivers are not meant to drive over them

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Islands

• Island Size and Designation
• Corner islands
• Urban 50 ft2
• Rural 75 ft2
• Length minimum 12 ft for each side
• Divisional islands
• Width 4 ft (6 ft if designed for pedestrians)
• Length 20 to 25 ft (high speed 100 ft)
• Delineation
• Curb height 6 in (low speed)
• May provide vegetation cover, mounted earth,
  shrubs for large islands
• Corner island offset and corner radii dependent
  on side length

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Islands
(see Ex. 9-41)
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Islands
(see Ex. 9-41)
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Islands
A Passenger cars occasional single-unit truck
B SU trucks occasional semi-trailers (WB-50)
(slight encroachment)
C Full access to WB-50
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Intersection Sight Distance

• The stopping sight distance needs to be provided
  at intersections
• Sight distance is provided to perceive the
  presence of potentially conflicting vehicles
• Thus, the driver need to have an unobstructed
  view of the entire intersection
• Rational a driver has enough distance to stop if
  a conflicting vehicle enters the intersection

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Intersection Sight Distance
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Intersection Sight Distance

• Characteristics
• Within the sight triangles, all obstructions
  should be removed buildings, parked vehicles,
  tress, hedges, tall grass, etc.
• Eye height 3.5 ft
• Object height 3.5 ft (4.35 ft less 10 inches
  allowance) (vehicle height)
• Sometimes, can use eye height for trucks (7.6 ft)

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Intersection Sight Distance

• Sight distance determine for different types of
  traffic control
• Case A no control
• Case B Stop controlled (2 stop signs)
• Case C Yield control
• Case D traffic signals
• Case E All-way stop controlled
• Case F left-turns from the major road

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Intersection Sight Distance

• Intersections with Stop Control (Case B)
• Case B1 left turns from minor road
• Case B2 Right turns from minor road
• Case B3 Crossing major road

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Intersection Sight DistanceCase B1
b
a
Assumptions Position of vehicle 14.5 ft from
edge of traveled way Left position of vehicle
½ lane (dimension a above) Right position of
vehicle 1 ½ lane (dimension a above)
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Intersection Sight DistanceCase B1
Dimension b in figure above
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Intersection Sight DistanceCase B1
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Intersection Sight DistanceCase B1
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Intersection Sight DistanceCase B1
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Intersection Sight DistanceCase B2
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Intersection Sight DistanceCase B2

• Right Turn from the Minor Road
• Use same approach as for left turn
• However, need to adjust for tg
• The time gaps can be reduced by 1 sec

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Intersection Sight DistanceCase B2
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Intersection Sight DistanceCase B2
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Intersection Sight DistanceCase B2
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The length needs to be adjusted by dividing the
total width of the lanes ( median width) to be
crossed by the sine of the intersection angle.
If the difference is higher than 12 ft, need to
adjust the computation of the sight distance by
adding an additional lane (e.g., exhibits 9-54
9-57).
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Median Openings

• General Characteristics
• Design opening and median ends should be based on
  traffic volumes, urban/rural area
  characteristics, and type of turning vehicles
• Should try to minimize encroachments on adjacent
  lanes or on the median itself
• Urban intersections have been found to be
  operating more safely with narrow median widths
• Rural unsignalized intersections operate more
  safely with wider median widths

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Equivalent right-turn radii
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Median Opening (Design Vehicle P)
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Median Opening (Design Vehicle P)
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Median Opening (Design Vehicle SU)
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Median Opening (Design Vehicle SU)
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Shape of Median End
Semicircle

• M less than 10 ft
• Not good for wider medians

M
Bullet Nose

• M wider than 10 ft
• Provide better guidance for the left turning
  vehicle

M
Example 40-ft, 50-ft or 70-ft radius
2-ft radius
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Median Opening (Skew)
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High-Speed / High Flow or High Left-Turn Movement
20 mph
25 mph
30 mph
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Indirect Left-Turns
400 600 ft
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Median Openings for U-Turns
Try to provide left-turn lane if median is wide
enough
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Speed-Change Lane
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Speed-Change Lane

• General Characteristics
• Are required for high-speed or high-volume roads
  where a change in speed is necessary for vehicles
  exiting and entering the through-traffic lanes
• All drivers do not use them in the same manner
  some use little of the available facility
• Deceleration lanes on the approaches to
  intersections can also be used for storage lanes
  for turning traffic

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Speed-Change Lane
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Speed-Change Lane
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Speed-Change Lane
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Speed-Change Lane
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