Roadside

CE 453 Lecture 23

Sideslopes – Foreslope (Backslope) Design

1. Considerations: Stability and Vehicle Recovery

a. if slope “>” 3:1 use barrier (if in clear zone)b. Hinge Point – minimize to avoid airborne vehiclec. Foreslope – flat enough to allow recovery (4:1 or

less desired)d. Toe of Slope – round to avoid plowinge. Ditch Bottomf. Backslope (3:1 or “less” desired)

2. AASHTO RDG has preferred cross section for two types of channels

Source: A Policy on Geometric Design of Highways and Streets (The Green Book). Washington, DC. American Association of State Highway and Transportation Officials, 2001 4th Ed.

Recoverable slopes

motorists are able to stop/slow and return to the roadway safely

1 to 4 or flatter

Recoverable Slope (unless you hit the sign)

<www.geocities.com/Colosseum/Slope/7683/site/highway/10-1.jpg>, February 28, 2002

Non-recoverable, traversable slopesmotorists unable to stop or return to

roadway safelybetween 1:3 and 1:4 recovery area must be to the toe of the

slope and a clear runout area at base with 1:6 or flatter slope

Source: A Policy on Geometric Design of Highways and Streets (The Green Book). Washington, DC. American Association of State Highway and Transportation Officials, 2001 4th Ed.

Critical slopes

vehicle likely to overturn

Non-Recoverable Slope

http://ppihc.artemisimages.com/searchres.asp?appmode=detail&id=ppih0750&kw=Scenic&page=1&ec=&collection=&spec=

Roadside – Components

1. Urban Areasa. Curbb. Horizontal Clearancec. Buffer to Land Used. Walkways

2. Rural Areas (focus)a. Clear Zone Conceptb. Drainage Channel Designc. Traffic Barrier Used. AASHTO Road Design Guide (RDG)

Roadside Design Guide

American Association of State Highway and Transportation Officials (2002)

Guide to address design issues off the traveled way (traffic lanes)

Roadside1. Clear Zone (def.) – “The total roadside

border area, starting at the edge of the traveled way, available for safe use by errant vehicles”

2. Concept – area should be as wide, flat, and rounded as practical and as free as practical from physical obstructions (want a driver who leaves roadway to regain control and return to the roadway)

Roadside3. AASHTO RDG

a. 30 feet (9 m) – 80 to 85 % recovery

b. Clear zone recommended = f(design speed, ADT, roadside slope, curvature)

c. Compromise between “absolute” safety and engineering, environmental, and economic constraints (always)

Roadside Design Options for Objects

1. Remove (REDESIGN)

2. Relocate

3. Reduce impact severity (breakaway)

4. Redirect by shielding

5. Delineate

But what is an object? signs, poles, slopes

Clear Zone

Unobstructed, relatively flat area beyond the edge of the traveled way

Provides area for vehicles to recover Width depends on:

Volume Design speed Slope of embankment

Measured from edge of pavement to nearest Obstacle (tree, telephone pole, etc) Steep slope, non-traversable ditch Cliff Body of water

http://www.storycounty.com/engineer/roadside_safety.htm

Clear Zone

Source: Garber and Hoel

http://epdfiles.engr.wisc.edu/pdf_web_files/tic/other/SAFERcontent_96.pdf

before

after

Iowa’s Roadway-Related Fatal Crashes

52% of Iowa’s fatalities are related to Lane Departure

39% of Iowa’s fatal crashes are single-vehicleRun-Off-the-Road (ROR) crashes

What would clear zone be for same scenario with cut slope instead?

What would clear zone be for same scenario with cut slope instead?

23 feet vs. 30 feet

Why???

Traffic Barriers

Purpose is to minimize severity of potential accidents when vehicles leave the roadway

Prevents vehicles from striking an object such as tree or bridge abutment

Are obstacles themselves (need to consider use)

http://www.southernguardrail.com/p_highway_gr_ar_01.html

http://www.southernguardrail.com/p_highway_gr_ar_01.html