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)

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

Highway functional classification

Mobility

Access

Principal arterials

Minor arterials

Major collectors

Minor collectors

Local roads and streets

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.

Provo example – graphical presentation

Principal arterial

Minor arterial

Collector

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

Rural roads example – graphical presentation

Principal arterial

Minor arterial

Major collector

Minor collector

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

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

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

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.

Cross section elements

Cross slopes

High-type pavements: 1.5% to 2.0%

Intermediate-type pavements: 1.5% to 3%

For quicker drainage.

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

Roadside design

“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

Breakaway sign posts

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