tte4004%20Chapter%202%2C7 - Intersection Design...

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Unformatted text preview: Intersection Design Intersection The efficiency and safety of a highway depend largely on its intersections. Types of intersections: – intersections at grade – grade separations without ramps – interchanges General Design Consideration General Design Objectives • reduce the potential and severity of collisions, and • facilitate convenient and comfortable traversing the intersection. Basic Elements of Design • Human factors: driving habits, drivers' expectancy, decision and reaction time, natural paths of movement, pedestrians and bikes General Design Consideration General •Traffic: capacity, characteristics of vehicles, vehicle speeds, accident experience • Physical elements: vertical alignment, sight distance, conflict area, traffic control devices, safety features • Economic factors: cost of improvements, effect of access control, energy consumption Intersection Functional Area Intersection T Intersections Intersections Exhibit 9-5 Exhibit 9-6 T Intersections Intersections Exhibit 9-7 Exhibit 9-8 Four-Leg Intersections Four Exhibit 9-9 Four-Leg Intersections Four Exhibit 9-11 Exhibit 9-12 Roundabouts Roundabouts Highway Interchanges Highway An interchange is a gradeAn separated intersection with ramps to connect them. Traffic signals may be installed at the ends of the ramps, traffic on the surface street (blue) also flows smoother without interference from the freeway. Diamond Interchange Diamond The design of choice for The lower-traffic interchanges. Traffic signals can be installed at the two points where the ramps meet the surface street. If a ramp also has the task of storing queued-up traffic, its length becomes a critical design factor. Cloverleaf Interchanges Cloverleaf The cloverleaf allows The "non-stop" full access between two busy roads. Traffic merges and weaves, but does not cross at-grade. Typically a cloverleaf is used where a freeway intersects a busy surface street. There are full, partial cloverleaf designs with and without CD Road Trumpet Trumpet This is a conceptually This simple way to end one freeway at another. Like a cloverleaf, it requires only one or two bridges, and designing for higher speed will take up more land. The Stack The For the driver, the 4-level For stack is the nice choice for two intersecting freeways. Each road has a direct connection to the other roadways, with no looping or weaving, and the ramps cross in a 4-level deck you can see for about a mile. If the ramps are two lanes wide, the interchange has quite high capacity. Single Point Urban Interchange (SPUI) Single SPUI is a relatively new SPUI variant of the diamond. It’s advantages include: – Compact layout; requires less right-of-way acquisition – Allows concurrent left turns for greater capacity The biggest disadvantage is The its Complexity and signal phases may be unfamiliar to drivers Alignment at Intersections Alignment Exhibit 9-18 Vertical Alignment at Intersections Vertical •Use grades 3% or flatter; maximum is 6% •Grade-line of the major highway should not be affected •A plane at an intersection is allowed at minor intersections •Avoid superelevation •Provide adequate sight distance Turning Roadways Turning Design of turning roadways is presented in lecture: Horizontal Alignment (3). Design cases • Minimum design • With corner island • Free-flow design Turning Roadways Turning Minimum Design Offset Exhibit 9-21 Sharpest turns Unchannelized turning paths Low speeds (<15 km/h) See Design Controls and Criteria (1) Applies also to left-turn paths Turning Roadways Turning Minimum Design Turning Roadways Turning Minimum Design ...
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