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10/23/2012
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Intersections & Interchanges
Intersections Types & Definition
• Grade‐separated without ramps
• Interchanges (grade separated with ramps)
• At‐gradeIntersection: Two or more streets join or crossat‐grade.The intersection includes the areas needed forThe intersection includes the areas needed forall modes of travel: pedestrian, bicycle, motorvehicle, 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: www\fhwa\Flexibility 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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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: www\fhwa\Flexibility 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 edgeRural 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 LanesTapers
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 completethe 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 yobstruction
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 -- b b
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Large Tree
72’
Example
25 mph
47’
45 mph
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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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da = 220 feet
Large Tree
b = 72’
Example
25 mph
db
a = 47’
45 mph
b
d
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45 mph
ddaa = 220 feet= 220 feetddbb = a __= a __ddaa__ = __ = 47’ (220’)47’ (220’) = = 69.9’69.9’
ddaa –– b 220’ b 220’ –– 72’72’
da
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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 sight 25 mph > 15 mph, stopping sight distance is not sufficient for distance 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
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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 turnsda : similar to da for stopda : similar to da for stop--controlled but increase controlled 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
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bottom 2 feet are painted orange) Target Rod
Measuring at an Uncontrolled Intersection
Assistant
Obstruction
X
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ObserverDecisionPoint
Obstruction
Y
