Module 7: Navigating Highway Interchanges Flashcards
Elements of an Interchange.
- Exit Terminal - speed change lane
- Ramp
- Entrance Terminal - speed change lane
Basic Design Principles I.
-System considerations
-not only the design of each individual interchange
-it must be considered in conjunction with the design of adjacent
interchanges
-Freeway & interchange design closely linked
-important to integrate designs as one
-treat as a whole system
-signing critical: human factors
Basic Design Principles (II).
-Uniformity of patterns & design consistency
-route continuity
-uniformity of signing
-entrance & exit right
-Single exit
-one exit per roadway per direction
-reduces driver workload
-some exception (high volume, high speed)
Basic Design Principles III.
-Loop ramps
-for exiting traffic from lower-speed roadways
-not generally desirable for freeway exits
-Return movements
-provide ramps to serve all directions
-Over vs Under
-advantages of freeway under
The Complexity of Interchange Design.
The complexity:
1. Variety of design elements that are present
2. Intricacy of traffic operations on those elements
3. Impact that design decisions revolving around these elements have upon the users of the facility.
The Impacts of Driver Workload.
-At freeway interchange locations, workload increases substantially for all drivers
-Interchange driving tasks are substantial & relatively risky. Example, exiting the freeway:
-search for and respond to guide signs
-identify interchange area
-identify correct exit lane
-search for a gap & change lanes
-take exit
-reduce speed according to off-ramp requirements
-monitor any ramp vehicles directly ahead
-follow the off-ramp path
-adhere to traffic controls/signals
Collision Rates at Interchange Areas.
-Collisions are far more likely to occur here than on mainline freeway sections between interchanges.
-Research on collision rates on ramps
-vary significantly depending on the ramp type being used
-on-ramps had a lower collision rate than off ramps
-the rates tended to increase as interchange spacing decreased
Interchange Warrants.
I. Classification: freeways require interchanges
II. Traffic: capacity limitations of at-grade intersection
III. Topography: may preclude an at-grade
IV. Safety: some unusual locations
V. User Benefits: reduce delay
Location & Spacing Principles.
-Location - serves communities
-Spacing guidance:
-urban: 2-3km
-rural: 3-8km
-Collision rates increase with decreased interchange spacing in urban areas.
Interchange Coordination - Left Exits/Entrances.
Issues:
-violates design consistency principle
-create hesitant/erratic operation
-increases driver workload
-much higher collision rates than right side
Interchange Coordination - Exit Location.
-Exit locations are high collision areas
-Exit ahead of structure is more desirable
-Single exit per direction desirable
-Need for consistency throughout a system
Interchange Coordination - Basic Lanes.
Maintenance of Basic Lanes:
-Basic number of lanes = a minimum number of lanes designated
& maintained over a significant length of route
-Auxiliary lane = the portion of the roadway adjoining the highway
for speed change, turning, storage for turning, weaving, truck
climbing, & other purposes supplementary to through-traffic
movement
Interchange Coordination - Lane Balance.
-Lane balance refers to certain principles which apply at freeway exits & entrances.
-Exit: Nc = Nf + Ne - 1
-Entrance: Max: Nc = Nf + Ne
Min: Nc = Nf + Ne - 1
Interchange Coordination - Weaving Section.
-A weaving section is a length of one-way roadway where vehicles are crossing paths, changing lanes, or merging with through traffic as they enter or exit a freeway or collector road.
-Weaving sections occur frequently along freeways in urban areas. They are also found between ramps closely spaced, successive interchanges.
-Short weaving distance can cause considerable turbulence
Interchange Types.
-System (Freeway-Freeway) & Service (Freeway-Roads) Interchanges
-4 leg & 3 leg Freeway-Freeway interchanges
-Types:
Diamond
Parclo A
Parclo B
Parclo AB, Rotary, Trumpet
Freeway-Freeway
Characteristics of a Diamond.
Advantages:
-Economical in property use & construction costs
-Single exit feature simplifies signing of freeway
-No need for speed change lanes or under the structure
Disadvantages:
-Lower capacity on the minor road due to left turn movements
-Potential restricted sight distance at exit ramp terminals especially where the minor road crosses over the freeway.
-Many points of conflict on the minor road increase the accident potential of the design, unless signalized.
Characteristic of Parclo A4.
Advantages:
-Single exit features simplifies signing
-High capacity
-All traffic movements are natural
Disadvantages:
-Higher construction and property costs compared to Diamond
-Signals required on minor road when through and turning volumes are high
Characteristics of a Parclo A2.
Advantages:
-Less property impacts than Parclo A4
-Single exit features simplifies signing
-May be used to stage construction of parclo A4
Disadvantages:
-Natural right turn is replaced by a left turn from the minor road
-Points of conflict on the minor road at the ramp terminals limit capacity & safety
-Stop condition on the minor road for left turn movement
Characteristic of Parclo B4.
Advantages:
-Single exit features simplifies signing
-Ramp traffic entering crossing road does not stop
-All traffic movements are natural
Disadvantages:
-Higher construction and property costs compared to Diamond
-Turning traffic on crossing road to the freeway may need storage lane treatment
-High speed traffic must exit from freeway on a small radius loop
Characteristics of a Parclo B2.
Advantages:
-Single exit features simplifies signing
-May be used to stage construction of parclo B4
Disadvantages:
-Points of conflict on the minor road at the ramp terminals limit capacity & safety
-Right turn traffic from the freeway must come to a stop at the minor road
-Turning traffic on crossing road to the freeway may need storage lane treatment
-High speed traffic must exit from freeway on a small radius loop
Characteristics of a Parclo AB.
Advantages:
-Similar to Parclo A2/B2
Disadvantages:
-Similar to parclo A2/B2
-Weaving section on crossing road
