Chapter 19: Elements of Intersection Design and
Layout
Chapter objectives: By the end of this chapter the
student will be able to:
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Make an initial sizing of the intersection
Choose proper channelization design
Select proper alternatives to improve unsafe
intersections
Describe types of mounting signal heads
Whether signalized or unsignalized, the control measures
implemented at an intersection must be synergistic with the
design and layout of the intersection.
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What’s discussed in this chapter
includes techniques for:
Determining the appropriate number and
use of lanes at an intersection approach,
 Channelization,
 Right- and left-turn treatments,
 Special safety issues at intersections,
 Location of intersection signs and signal
displays.
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19.1 Intersection Design Objectives and
Considerations
Two primary objectives for intersection design:
1.
2.
To ensure safety for all users, including drivers,
passengers, pedestrians, bicyclists, and others
To promote efficient movement of all users (motorists,
pedestrians, bicyclists, etc.)
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The elements considered:
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Human factors (approach speed, user expectancy, decision and
reaction times, etc.
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Traffic considerations (appropriate capacity, distribution of vehicle
types and turning movements, approach speeds, special treatments for
transit vehicles, pedestrians, and bicycles)
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Physical elements (abutting properties, traffic movements generated by
them, intersection angle, existence and location of traffic control
devices, sight distances, specific geometric characteristics)
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Economic factors (cost of improvements, effects of improvements on
the values of abutting properties, the effect of improvements on energy
consumption
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Functional intersection area (operational intersection area including
deceleration and acceleration zones as well as queuing areas)
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19.2 A Basic Starting Point: Sizing the
Intersection (how many lanes do we need?)
19.2.1 Unsignalized intersections
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STOP or YIELD – see chapter 16
LT and RT lane considerations for Stop- or Yield-controlled
intersections.
Assumptions:
• All through
vehicles
• Capacity
80% of a lane
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19.2.2. Signalized intersections (see chapters 17 and 18)
• Check LT needs
• Determine proper phase
sequence
• Determine critical
movements
• Determine cycle length
and split effective green
time
1
 3,600 
Vc  3,600  Nt L 

h
 C 
Tab 19.3 Sample Prob.
Assumptions: h = 2.6 sec/veh
tL = 4 sec/phase
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Sample Problem
Vc
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19.3 Intersection Channelization
19.3.1 General principles
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Vehicle paths may be confined so that no more than two paths cross at
any one point
The angles at which merging, diverging, or weaving movements occur
may be controlled
Pavement area may be reduced, decreasing the tendency to wander and
narrowing the area of conflict between vehicle paths
Clear indications of proper vehicle paths may be provided
Predominant movements may be given priority
Areas for pedestrian refuge may be provided
Separate storage lanes may be provided to permit turning vehicles to
wait clear of through traffic lanes
Space may be provided for the mounting of traffic control devices in
more visible locations
Prohibited turns may be physically controlled
Vehicle speeds may be somewhat reduced
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Freedom & Bulldog
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19.3.2 Some examples and 13.3.3 Channelizing right turns
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Look at the examples shown in pages 545 and
547 and learn what you can do with
channelization to provide safer and smoother
turns
Make certain a RT bay is not blocked by
queued through vehicles. Queue length must be
estimated. Otherwise the RT bay will be useless
(but you see this example at many locations,
e.g., the RT lane on Bulldog WB at Taco Bell;
the LT lane on northbound on University
Parkway at Brent Brown car dealership
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19.4 Special Situations at Intersections
19.4.1 Intersections at skewed angles (Read carefully)
Existing
Potential realignment
Which one is better?
Alternative solution using
channelization
Realignment
Chapter 19 of 4-leg odd-angle intersection:
Remember two intersections are created.
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19.4.2 T-intersections: Opportunities for creativity
Layout and
phasing
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19.4.3 Offset intersections – Problems and solutions
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19.4.4 Special treatments for heavy LT movements –
Think of advantages and disadvantages of these
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LT Lanes
Continuous
flow
intersection
http://www.abmb.com/cfi.html
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The Advantages of CFI
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Simulations using VISSIM traffic modeling software show that CFI
outperforms conventional alternatives dramatically. In case after case,
CFI produced extraordinary improvements in levels of service under
existing traffic loads and reductions in average intersection delay of
90% or more.
Achieves significantly more capacity than conventional at-grade
intersection designs.
Offers substantial savings over grade-separated alternatives while
providing equal or better performance.
Can be deployed in one-, two-, three-, and four-legged versions.
Requires little more right-of-way than a conventional at-grade
intersection.
Provides clear lines of sight, avoiding the visual barriers created by
overpasses.
Helps reduce pollution by reducing congestion.
Requires shorter construction time and less utility relocation.
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Diverging Diamond Interchange
American Fork Main St.
Interchange proposed design.
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19.5 Street Hardware for Signalized
Intersections
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At least one of the two required
signal faces for the major
movement must be located
between 40 and 150 ft of the
STOP line, unless the physical
design of the intersection
prevents it.
All signal faces should be
placed within a horizontal 20º
angle around the centerline of
the intersection approach
(including exclusive left- and/or
right-turn lanes)
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All signal faces should be placed at mounting
heights in conformance with MUTCD standards
X-axis  related to street width
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Post-mounted
Mast-mounted
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Cable-mounted – flexible signal head arrangement
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