This document discusses different types of intersections and interchanges, including their key elements and design considerations. It describes at-grade intersections, channelization techniques, and different interchange configurations like diamonds, cloverleafs, and trumpets. The document also covers factors in curb radius, turn lane, and sight distance design, and provides examples of determining sufficient sight distance at intersections.

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Intersections & Interchanges
Intersections Types & Definition
• Grade‐separated without ramps
• Interchanges (grade separated with ramps)
• At‐grade
Intersection: Two or more streets join or cross
at‐grade.
The intersection includes the areas needed forThe intersection includes the areas needed for
all modes of travel: pedestrian, bicycle, motor
vehicle, and transit.

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At‐grade intersections
• All highways have intersections at grade
except freeways so that the intersection areaexcept freeways, so that the intersection area
is a part of every connecting road or street.
• In this area, crossing and turning movements
occur.
• Some intersection are channelized
– minimize traffic accidents, speed control,
prevention of prohibited turns, refuge may be
provided for pedestrians.
At‐grade Intersections Types
Unchannelized T
Unchannelized YUnchannelized Y
Flared T

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3‐leg intersections
Y with turning roadways
Unchannelized
Channelized
• Traffic circles
–Rotaries: large diameter > 300 ft, allows
At‐grade Intersections Types (Cont.)
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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Intersection: Key Elements
• Safety and efficiency
• Consider both vehicles and pedestrians
• Minimize severity of potential conflicts
• In general, these conflicts may be classified as:
M i fli t
Merging
Diverging
Basic Principles
– Merging conflicts
• Occurs when vehicles enter a traffic stream
– Diverging conflicts
• Occurs when vehicles leave the traffic stream
– Weaving conflicts
• Occurs by merging then diverging
– Crossing conflicts
• Occurs when they cross paths directly
Weaving
Crossing

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Crossing Conflicts Solutions
• Time‐sharing
• Space‐sharing
• Grade separation (Interchanges)
INTERCHANGES
• Are classified according to the way they
h dl l ft t i t ffihandle left‐turning traffic.
INTERCHANGE CONFIGURATION
‐ are selected on the basis of structural cost,
right of way costs and ability to serve trafficright‐of‐way costs, and ability to serve traffic.

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Diamond Interchange
Diamond Interchange
• Diamond Interchangeg
– Employ diamond ramps
which connect to the cross
road by means of an at
grade intersection.
– Left turns are accomplished
by having vehicles turn left
across traffic on the cross
road.road.

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Cloverleaf Interchange
 Cloverleaf InterchangeCloverleaf Interchange Cloverleaf InterchangeCloverleaf Interchange
 Employ Employ loop rampsloop ramps, in , in
which vehicles turn left which vehicles turn left
by turning 270 degrees by turning 270 degrees
to the right.to the right.

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Partial cloverleaf
 Partial Cloverleaf Interchange (Partial Cloverleaf Interchange (ParcloParclo))
 Involves various combinations of diamond and loop ramps.Involves various combinations of diamond and loop ramps.

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Trumpet
 Trumpet Interchange Trumpet Interchange

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Full Directional
Directional‐Y

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ON‐RAMP (entrance to highway)
ON‐RAMP (entrance to highway)
OFF‐RAMP (exit to highway)
OFF‐RAMP (exit to highway)
Intersection – Design Controls
Functional class of roadways
Topography and environment (manmade and
natural)
Design speed
Design vehicles
Traffic Characteristics (design volumes, level of
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Traffic Characteristics (design volumes, level of
service)

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Intersection Design 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
• Make adjustments away from intersection
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• Traffic lanes should be visible and obvious to
motorists
• Motorists should understand the path they are
supposed to take
Elements of Design
• Design of alignment
D i f h li t• Design of channeling system
• Determination of minimum required widths of
turning roadways
– Speeds > 15 mph
• Intersection sight distance
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• Determination of number of lanes
– Provision of turning lanes

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Alignment Horizontal
• 90° intersection of approaches
• Skewed
– Visibility
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– Longer crossing times in some cases
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 Design
• Factors:
– Design vehicle
– Intersection angle
– Approach width and parking
– Channelization
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– Pedestrians
– Allowable speed reduction
Design Vehicle
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Source:
wwwfhwaFlexibility
in Highway Design -
Chapter 8 - FHWA.htm

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Radius Design
• Simple curve
–Low speed collector, local streets
• Simple circle with taper
d d
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• 3‐centered compound curve
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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34

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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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with arterials

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fh Fl b l H h D
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Source: wwwfhwaFlexibility in Highway Design
- Chapter 8 - FHWA.htm
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
• Provides clear path for different movements
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• Provides pedestrian refuge
• Provides storage area for turning vehicles
• Controls prohibited turns
• Restricts speed
Types of Channelization
• Raised islands
– Urban
– Provides refuge for
pedestrians
– <= 50 ft2 in urban areas
– <= 75 ft2 in rural areas
P t ki
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• Pavement markings
– Low pedestrian volume,
low approach speeds

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Types of Channelization (Cont.)
• Pavement edge
Rural painted if high speed– 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 movementTurning movement
volumes
• Accident
experience
• Capacity
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Left & Right Turn Lane Design
• Number of likely queued vehiclesy q
– Type of control
– Number of turning vehicles
– Length of vehicles
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Deceleration Lanes
• Provides distance for turning vehicles to
d l t ith t i t f i ithdecelerate away without interfering with
through traffic
• Deceleration lane length depends on:
– Speed
– number of queued vehicles
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number of queued vehicles
– vehicle length
Auxiliary Lanes
Tapers
MEDIAN
MEDIAN

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Median Openings
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Provide median refuge to provide
crossing in stages
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Provision of crosswalks
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Adequate Sight Distance – ISD
 Allow drivers to have an unobstructed view of
intersection
D fi iti R i d ISD i th l th f d• Definition: Required ISD is the length of cross road
that must be visible such that the driver of a
turning/crossing vehicle can decide to and complete
the maneuver without conflict with vehicles
approaching the intersection on the cross road.
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Adequate Sight Distance – ISD
 Sight Triangle – area free of obstructions necessary
to complete maneuver and avoid collision – needed
for approach and departure (from stop sign forfor approach and departure (from stop sign for
example) – Exhibit 9‐50
 Allows driver to anticipate and avoid collisions
 Allows drivers of stopped vehicles enough view of
the intersection to decide when to enter
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Sight Triangle
 area free of obstructions necessary to
l t d id lli icomplete maneuver and avoid collision –
needed for approach and departure (from
stop sign for example)
 Consider horizontal as well as vertical, object
below driver eye height may not be an
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y g y
obstruction
 AASHTO assumes 3.5’ above roadway
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Sight Distance Obstruction
Hidd V hi lHidden Vehicle
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ISD Cases
• No control: vehicles adjust speed
• Stop control: where traffic on minor roadway must• Stop control: where traffic on minor roadway must
stop prior to entering major roadway
• Yield control: vehicles on minor roadway must yield
to major roadway traffic
• Signal control: where vehicles on all approaches are
required to stop by either a stop sign or traffic signal
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• All way stop
• Stopped major roadway left‐turn vehicles – must
yield to oncoming traffic

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Case A– No Control
• Rare? – Not really ‐ Iowa
• Minimum sight triangle sides = distance traveled in 3• Minimum sight triangle sides = distance traveled in 3
seconds (design or actual?) = 2 seconds for P/R and 1
second to actuate brake/accel.
• Assumes vehicles slow ~ 50% of midblock running
speed
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Case A– No Control
• Prefer appropriate SSD on both approaches
(minimum really)(minimum really)
• Provided on lightly traveled roadways
• Provide control if sight triangle not available
• Assumes vehicle on the left yields to vehicle on the
right if they arrive at same time
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Critical speed is set to stopping distance dCritical speed is set to stopping distance dbb = a __= a __ddaa____
ddaa -- bb

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Large
Tree
72’
Example
25 mph
47’
45 mph
61
45 mph
Is sufficient stopping sight
distance provided?
Large
Tree
b = 72’
Example
25 mph
db
a = 47’
50 mph
b
d
62
50 mph
ddbb = a __= a __ddaa____
ddaa -- bb
da

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63
da = 220 feet
Large
Tree
b = 72’
Example
25 mph
db
a = 47’
45 mph
b
d
64
45 mph
ddaa = 220 feet= 220 feet
ddbb = a __= a __ddaa__ =__ = 47’ (220’)47’ (220’) == 69.9’69.9’
ddaa –– b 220’b 220’ –– 72’72’
da

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65
db = 69.9 feet
corresponds to 15
mph
Large
Tree
b = 72’
Example
25 mph
db
a = 47’
45 mph
b
d
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45 mph
25 mph > 15 mph, stopping sight25 mph > 15 mph, stopping sight
distance is not sufficient fordistance is not sufficient for
25 mph25 mph
da

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Case B – Stop Control
Three Sub Cases – Maneuvers
• Turn left on to major roadway (clear traffic left enter• Turn left on to major roadway (clear traffic left, enter
traffic right)
• Turn right on to major roadway (enter traffic from
left)
• Crossing (clear traffic left/right)
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Case B – Stop Control
• Need ISD for departure and completion even if p p
vehicle comes into view at point of departure = 1.47
Vmajor * tg where tg=7.5‐11.5s; add more for grade
or multilane; decrease by 1s for right turns
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or multilane; decrease by 1s. for right turns

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Left
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turn
right turn
and crossing
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Case C ‐ Yield Control
• Minor Roadway Yields – must be able to see
left/right – adjust speed – possibly stop
• Sight distance exceeds that on stop control• Sight distance exceeds that on stop control
• Similar to no‐control
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Case C ‐ Yield Control
• Must use minimum stopping sight distances
for da and db rather than values from Table 7.7
(page 251, Garber and Hoel)(page 251, Garber and Hoel)
• SSD calculation should include effect of grade
• Required distance = P/R + stop
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Case C ‐ Yield Control
• Typically Known – a, b
• Typically Assume Va or Vb
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Case C ‐ Yield Control
• Typically Known – a, b
• Typically Assume Va or Vb
• Similar triangle can be used to calculate safeSimilar triangle can be used to calculate safe
approach speeds (given one approach speed) or
allowable a and b.
• da/db = (da – b)/a
• db = (da *a)/ (da – b)
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db
75
Critical speed is set to stopping distance dCritical speed is set to stopping distance dbb = (d= (daa *a)/ (d*a)/ (daa –– b)b)
da
Yield Control
• Case C I: Crossing maneuver from minor road
• Assumes that minor road vehicles that do not stop
decelerate to 60% of minor road speed
• Vehicle should be able to:
• Travel from decision point to intersection decelerating
to 60% of design speed
C d l th i t ti t th d
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• Cross and clear the intersection at the same speed

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tgg
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78Need tg

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Yield Control
• Case C: Left and Right turns at yield control
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db
da
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ddbb = 82 ft to accommodate left and right turns= 82 ft to accommodate left and right turns
da : similar to da for stopda : similar to da for stop--controlled but increasecontrolled but increase
time gaps by 0.5 sectime gaps by 0.5 sec
da:
length
of major
approach
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• Case D: Signal control
First vehicle stopped should be visible to driver of
other approaches
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Also …
• Case E: All way stop
• Case F: Left turn from major
• tg=5.5‐7.5s + multilane adjustmentg j
• Effect of Skew
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Sighting Rod and Target Rod
(AASHTO)
• For vertical sight
distance with verticaldistance with vertical
curves
• Sighting rod‐ 3.5 feet
tall
• Target rod‐ 4.25 feet tall
(Top portion and
b f
Sighting
Rod
85
bottom 2 feet are
painted orange) Target Rod
Measuring at an Uncontrolled
Intersection
Assistant
Obstruction
X
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Observer
Decision
Point
Obstruction
Y