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 vehicle c.Foreslope – flat enough to allow recovery (4:1 or less desired) d.Toe of Slope – round to avoid plowing e.Ditch Bottom f.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, th 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), February 28, 2002

Non-recoverable, traversable slopes motorists unable to stop or return to roadway safely between 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, th Ed.

Critical slopes vehicle likely to overturn

Non-Recoverable Slope

Roadside – Components 1.Urban Areas a.Curb b.Horizontal Clearance c.Buffer to Land Use d.Walkways 2.Rural Areas (focus) a.Clear Zone Concept b.Drainage Channel Design c.Traffic Barrier Use d.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)

Roadside 1.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)

Roadside 3.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

Clear Zone Source: Garber and Hoel

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-vehicle Run-Off-the-Road (ROR) crashes

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)