lec 21, ch.16, pp.671-688: highway design standards (objectives) understand how highways are...
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Lec 21, Ch.16, pp.671-688: Highway design standards (Objectives)
Understand how highways are functionally classified
Learn what factors are considered in a typical highway design
Learn design standards for major parts of highways
Learn the elements of highways (mostly from reading)
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What we cover in class today…
Functional systems of urban roads and rural roads
Factors influencing highway design Design standards: (once after target
functional class is determined) Design hourly volume, Design speeds, Elements of cross section
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Highway functional classification
Mobility
Access
Principal arterials
Minor arterials
Major collectors
Minor collectors
Local roads and streets
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Functional system of urban roadsSmall urban areas: Population 5,000 to 50,000 Large urban areas: Population 50,000 or more
Urban principal arterials
Serves the major activity centers: Interstate, expressways, other principal arterials
I-15, State St., University Ave., University Parkway, 900E, Center St.
Urban minor arterials
Interconnect with and augment principal arterials
Bulldog Ave (1230N), Freedom Blv., Geneva Rd.
Urban collector streets
Collect traffic from local streets and convey it to the arterial system
Canyon Rd., Timpview, 500N, 800N
Urban local streets
All other streets That’s where your house is.
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Provo example – graphical presentation
Principal arterial
Minor arterial
Collector
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Functional system of rural roads
Principal arterials
Serves mostly interstate trips and a substantial amount of intrastate trips
I-15, I-70, I-80, I-215, US89, US40, US6, US191, etc.
Minor arterials
Augments the principal arterials, connects cities, large towns, other large traffic generators
SR73,10, 130, 257, 56, etc.
Major collectors
Carry traffic to and from county seats and large cities not served by the arterial system
SR21, 24, 95, 18, 12, etc.
Minor collectors
Collects traffic from local roads and convey it to other facilities
SR174, 124, 96, etc.
Local roads
All roads within the rural area not classified above
All others
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Rural roads example – graphical presentation
Principal arterial
Minor arterial
Major collector
Minor collector
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Factors influencing highway design(Think of the Legacy Parkway in terms of these factors)
Functional classification
Expected traffic volume & traffic mix
Design speed
Topography
Level of service targeted
Available funds
Safety
Social and environmental factors
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Guidelines for selection of design LOS(AASHTO: A Policy on Geometric Design of Highways and Streets, 2000)
Functional Class
Area & Terrain Type
Rural level
Rural rolling
Rural Mtns
Urban & Suberban
Freeway B B C C
Arterial B B C C
Collector C C D D
Local D D D D
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Design hourly volumeIf we design a highway for the highest peak hour volume in the year, the highway will be too expensive. Researchers found that the 30th highest hourly volume would be a comfortable compromise.
This is the so-called “K” value.
DHV = AADT*K*D where D is the directional factor
Rural K=0.12-0.18, Urban K=0.08-0.10
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Design vehicleThe design vehicle is that vehicle selected to represent all vehicles on the highway. Weight, dimensions, and operating characteristics. Its turning characteristics are very important for intersection and turning roadway design.
See Chapter 3.
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Cross section elements
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Cross slopes
High-type pavements: 1.5% to 2.0%
Intermediate-type pavements: 1.5% to 3%
For quicker drainage.
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Barriers
A typical roadside barrier (with guardrail…)
Without guardrail…(Top two pictures taken from the WPI website)
Conc. Median barrier
Barrier and mountable curbs
Click this picture for a simulation
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Roadside design
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“Forgiving” highway concept
By improved design of the geometry of highway or by the use of various equipment, highways were made safer. These equipment reduce the severity of impact or allow the driver to steer back to the traveling lane.
(Pictures extracted from WPI’s web page, Worcester Polytech Institute, MA)
* Shock Absorbers at the gore area
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Breakaway sign posts
POZLOC.com