Module 8: Intersection Planning Flashcards
What is an Intersection?
-The area where two or more roads join or cross at-grade.
Complexity of Intersections.
-Intersections are the most challenging aspect of street design in an urban environment.
-The performance of an intersection affects:
-safety
-traffic flow & capacity of the road
Intersection Configurations.
-Three leg (T)
-Four leg
-Multi leg
-Roundabout
Design Considerations.
Four basic elements of design considerations:
-Traffic factors
-Physical factors
-Human factors
-Economic factors
Traffic Factors.
-Safety
-Collision history
-Turning movement volumes
-Pedestrian & bicycle volumes & movements
-Physical & operational characteristics of vehicles
-Special truck operation
-Time & duration of peak volumes
-Design capacities for local conditions, etc.
Physical Factors.
-Road classification - basic lane requirements
-Angle of intersection/intersecting road alignments
-Grades & sight distances
-Surrounding developments
-Intersection spacing & access management
-Roadside drainage
-Topography. etc.
Human Factors.
-User expectations
-User habits
-Perception & reaction times
-Sight line & distance
-Paths of movement
-Driver distraction
-Accommodate all road users
Economic Factors.
-Land cost
-Construction cost
-Utility cost
-Maintenance cost
-Cost effectiveness of alternative designs
-Staging of development
-Effect on local businesses & properties
Heavy Workload at Intersections.
The tasks in the process of negotiating any intersection:
-detect the intersection
-identify right of way, signalization, & appropriate paths
-Search for vehicles, pedestrians, & cyclists on a conflicting path
-Yield the right of way to various road users as appropriate
-Assess adequacy of gaps for crossing/turning movements
-Successfully complete through or crossing/turning maneuvers
Visual Search.
-Time available for search is limited by the demands of the driving task.
-Improper lookout, examples:
-right turning drivers ignore/overlook the search to the right for
vulnerable road users
-pedestrians look less for vehicles coming from the side & from
ahead
-More challenging at night
Visual Blockage.
-A high % of improper lookout is due to visual blockage
-Obstructions - newspaper boxes, utility poles, signs
-A critical view blockage - LT vehicles in opposing turn bay
Accommodation All Road Users.
-Consider the needs & specific limitations of all road users:
-drivers
-cyclists
-pedestrians
-The design should:
-provide adequate sight distance
-provide advanced positive guidance as needed, particularly where
lane drops occur
-be consistent & meet drivers expectations
Design - Maintaining a Balance.
-Design needs to accommodate a balance between the needs of all road users
-Depends on the volumes & priority given to the user
-Urban (active transportation) vs Suburban (cars & trucks)
Intersection Design - Operational Requirements.
-Provide adequate sight distance - for approach& departure maneuvers
-Minimize turning & through conflicts
-Avoid complicated geometry that increase driving workload & impacts acceleration/deceleration
Traffic Maneuvers.
-Diverging
-Merging
-Crossing
-Weaving
Vehicle Conflicts.
-A traffic conflict occurs whenever the paths vehicles follow diverge, merge, or cross
-The potential of traffic conflicts at intersections depends on the:
-number of one way pr two way approaches at the intersection
-number of lanes at each approach
-signal control
-traffic volumes
-percentage of right or left turns
Conflicts at a 3-legged Intersection.
-3 diverging
-3 merging
-3 crossing
-9 total conflict points
Conflicts at a 4-legged Intersection.
-8 diverging
-8 merging
-16 crossing
-32 total conflict points
Reduce Conflicts - The Two Dimensions.
Space:
-mitigate conflicts by physical design
-2D - intersection treatments & innovative intersection types
-3D - interchanges
Time:
-reduce number of conflict points by time sharing the same space
-intersection control
“Space” Strategies to Mitigate Collisions.
-Intersection treatments.
-Innovative intersections
-Interchanges
Intersection Treatments I.
-Decreases the relative velocities of the vehicles
-Decreases the relative angle between the vehicles
-Increases the number of lanes
-Adds speed-change lane
-No easy way for crossing conflicts
Intersection Treatments II (2D Separation).
-Pavement markings
-Signing to show prohibited turns
-Design to physically block or discourage certain moves
Why Innovation - Growing Challenges.
-Growing limitations on mobility
-growing traffic demand
-limited resources
-constraints on space
-community expectations changing
-Innovative tratments
-may overcome some challenges
Alternative Intersection Treatment.
-Displaced left turn
-Median U-turn
-Restricted crossing U-turn
-Quadrant roadway intersection
3D Separation - Interchanges.
-Conventional
-Innovative
-further reduce conflict points (e.g. diverging diamond)
Levels of Intersection Control.
-L1 Passive Control: Basic rules of the road
-L2 Semi-Control: Direct assignment of right of way using YIELD or STOP
-L3 Active Control: Traffic signalization
Level 1 Control - Basic Rules of the Road.
-This is used when the traffic coming to an intersection is low
-In order to safely operate under basic rules of the road, drivers on conflicting approaches must be able to see each other in time to assess whether an “impending hazard” is present, & to take appropriate action to avoid a collision
-Sight distances must be analyzed to ensure that they are sufficient for drivers to judge & avoid conflicts
Intersection Sight Distance.
-To allow drivers to have an unobstructed view of an intersection
-Important to perceive potential conflicts & to react
-Definition: “The designer should provide sight distance of sufficient length that drivers can control the operation of their vehicles to avoid striking an unexpected object in the traveled way.”
-Sight distance requirements must be considered both for approaching the intersection & departing from the stopped position at the intersection.
-It is defined as the sight distance available from a point where vehicles are required to stop on the intersecting road
Adequate Sight Distance is a Must for L1.
-dA & dB - minimum distances to avoid a crash
-Adequate sight distance is a necessary, but not sufficient, condition for adopting a “no control” option
-Traffic volumes or other conditions may make a higher level of control desirable
Approach Sight Triangle.
-Provides sight distance along intersection approach legs to allow drivers to view potential conflicting vehicles/objects on the intersecting roadway, & complete a safe stopping maneuver.
Departure Sight Triangle.
-Provides sight distance sufficient for a stopped driver on a minor road approach to depart from the intersection & enter or cross the major road.
Design Dimensions of Sight Triangles.
The dimensions vary with the type of traffic control used at an intersection:
-Case A - Intersections with no control
-Case B - Intersections with stop control on the minor road
-Case C - Intersections with yield control on the minor road
-Case D - Intersections with traffic signal control
-Case E - Intersections with all-way stop control
-Case F - Left turns from the major road
Level 2 Control - YIELD & STOP.
-The manual on uniform traffic control devices gives some general advice for the imposition of either STOP or YIELD signs
-Types
-YIELD control of minor approaches
-STOP control of minor approaches
-Multiway STOP control of all approaches
Humboldt Broncos Crash Case Study - Intersection Improvements.
-Sight triangle (remove trees)
-Advanced guidance (“stop ahead”)
-Rumble strips on Highway 335
-Wider shoulders
-Simplify the intersection - rearrange the overhead power lines, removal & realign nearby access roads
Level 3 Control - Traffic Control Signals.
-Active control implies that the road user will be forced to follow the path suggested by the traffic control agencies. The road user cannot maneuver according to his/her wish.
-It alternately assigns right of way to specific movements; it can substantially reduce the number and nature of intersection conflicts as no other form of control can.
-Traffic signals introduce a fixed source of delay into the system, it is important that they not be overused.
Traffic Signal Control - Advantages.
-Provides for the orderly movement of traffic
-Promotes driver confidence
-Increases the traffic handling capacity
-Reduces the frequency & severity of certain types of crashes
-Interrupts heavy traffic at intervals to permit other traffic, vehicular or pedestrian, to cross
-If coordinated, provides for continuous or nearly continuous movement at a definite speed along a given route under favourable conditions.
Traffic Signal Control - Disadvantages.
Poor design can lead to:
-Excessive delay
-Increase in fuel consumption
-Excessive disobedience of the signal indications
-Significant increases in the frequency of collisions (rear-end)
-Diversion to less adequate routes
Traffic Signal Warrants.
-Traffic Operations
-Justification 1 - minimum 8 hour vehicle volume
-Justification 2 - delay to cross traffic
-Justification 3 - combination warrant
-Justification 4 - minimum 4 hour volume
-Collision History
-Justification 5 - collision warrant
-Pedestrian Safety
-Justification 6 - pedestrian volume & delay warrant
Traffic Signal Warrants - Basic Meaning.
-No signal should be placed without an engineering study showing that the criteria of at least one of the warrants are met
-On the other hand, meeting one or more of these warrants does not necessitate signalization
-Not a mandatory standard, it calls only for consideration, not placement, of a traffic signal
-The engineering study needs to demonstrate that installation of a signal will improve safety, increase capacity, or improve the efficiency of operation.
-In the end, engineering judgement is called for, as it is appropriate in any professional practice
Traffic Signal - Types of Control.
-Pre-timed: A pretimed controlled is one that operates within a fixed cycle length using preset intervals & no detection.
-Semi-actuated: A traffic control signal that makes use of detection on some approaches to respond to vehicle and/or pedestrian demand.
-Full-actuated: A traffic control signal that makes use of detection on all approaches to respond to vehicle and/or pedestrian demand.
Signal Cycles.
-A traffic signal cycle is one complete set of signal phase sets
-The diagram shows a traffic signal cycle with 6 phases & 3 phase sets
-Each arrow represents a phase & each box represents a phase set
Signal Cycles - Phasing.
Left Turns:
-Permitted left turns: one that is made across an opposing flow of vehicles
-Protected left turns: one that is made without an opposing vehicular flow
-Compound left turns: left turns are protected for a portion of the signal cycle and permitted in another portion of the cycle
Phase Diagram.
-A phase is a timing unit that controls one or more movements
-Left turns are designated with odd numbers
-Through movements are designated as even number
Cycle.
-One complete rotation through all of the indications provided.
Cycle Length (C).
-The time (sec) it takes a signal to complete one full cycle of indications.
Green Interval (Gi).
-The actual duration the green light of a traffic signal is turned on.
Red Interval (Ri).
-The actual duration the red light of a traffic signal is turned on.
Lost Time.
-The time during which the intersection is not effectively used for any movement.
Phase.
-The green interval plus the change & clearance intervals that follow it.
Cycle Time.
-If cycle time is too short, the proportion of lost time in the cycle time is high making the signal control inefficient & causing lengthy delays.
-If the cycle time is too long, then waiting vehicles will clear the stop line during the early part of the green period.
Lost Time.
-Lost time is the time during which the intersection is not effectively used for any movement.
-Lost time per phase = tL = L1 + L2
-Start up lost time L1 = the portion of the green time that is not completely utilized
-Clearance lost time L2 = the last second of the yellow interval
Effective Green Time.
–The time during which a traffic movement is effectively utilized the intersection
-g = G + Y - tL
Effective Red TIme.
-The time during which a traffic movement is not effectively utilized the intersection.
- r = C - g
Intersection Design Principles - Summary.
-Simplify geometry
-Reduce conflict points
-Facilitate visibility & predictability for all users
Intersection Design Strategies.
-Design compact intersections
-Analyze intersections as part of a network, not in isolation
-Integrate time & space
-Intersections are shared spaces
-Utilize excess space as public space
-Design for the future
