Lec 21, Ch.16, pp.671-688: Highway design standards (Objectives)

1. 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)

2. 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

3. Highway functional classification Principal arterials Mobility Minor arterials Major collectors Minor collectors Access Local roads and streets

4. Functional system of urban roads Small urban areas: Population 5,000 to 50,000 Large urban areas: Population 50,000 or more

5. Provo example – graphical presentation Principal arterial Collector Minor arterial

6. Functional system of rural roads

7. Rural roads example – graphical presentation Principal arterial Major collector Minor arterial Minor collector

8. Factors influencing highway design(Think of the Legacy Parkway in terms of these factors) Functional classification Level of service targeted Expected traffic volume & traffic mix Available funds Design speed Safety Topography Social and environmental factors

9. Guidelines for selection of design LOS (AASHTO: A Policy on Geometric Design of Highways and Streets, 2000)

10. Design hourly volume If 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

12. Design vehicle The 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.

13. Cross section elements

14. Cross slopes For quicker drainage. High-type pavements: 1.5% to 2.0% Intermediate-type pavements: 1.5% to 3%

15. Barriers Click this picture for a simulation Without guardrail… (Top two pictures taken from the WPI website) A typical roadside barrier (with guardrail…) Barrier and mountable curbs Conc. Median barrier

16. Roadside design

17. “Forgiving” highway concept * Shock Absorbers at the gore area 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)

18. Breakaway sign posts POZLOC.com