Slope Flattening

DEFINITION AND DESCRIPTION

Slope flattening reduces the steepness of the side slope and depressed median so that errant motorists are more able to keep their vehicles stable, regain control, and avoid obstacles. Slopes of 1V:4H or flatter are considered recoverable (i.e., motorists can retain control of a vehicle by slowing or stopping). Slopes between 1V:3H and 1V:4H are generally considered traversable, but non-recoverable (i.e., the errant vehicle will continue to the bottom of the slope).

Foreslopes, or fill slopes, are parallel to the flow of traffic and are the front or negative slopes that usually hinge from the roadway’s shoulder. The backslope is a positive slope that extends upward from the base of the foreslope or toe of the ditch. Transverse slopes are not parallel to the roadway and are usually created by median crossovers, berms, driveways, or intersecting roadways. 

Implementation Categories

CONSIDERATIONS

• Slope flattening can improve fill slope stability.
• Right-of-way or space constraints may restrict usage.
• When restrictions prevent slope flattening, roadside barriers are an option to shield steep embankments.
• See AASHTO’s Roadside Design Guide (RDG) for more information on recoverable slopes and clear zone distances.
• Can be done on new construction or reconstruction projects or at site-specific locations (e.g., outside of curves) to improve safety.
• On large earthwork projects with excess material, a false ditch in steep embankment areas provides a wider recovery area.

APPLICATION

• When feasible, design or correct side slopes with recoverable or traversable foreslope and back slope gradients the width of the clear zone. Slopes of 1V:4H or flatter are preferred for foreslopes.
• Transverse slopes of 1V:6H or flatter are recommended, particularly for sections immediately adjacent to traffic. On high-speed roadways transverse slopes are often 1V:10H or flatter.
• In clear zones, include traversable headwalls unless the headwall is protected by a barrier. See KYTC’s Drainage Manual (DR-606.5) for additional information on safety headwalls and their applications.

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Complimentary SSA Strategies

• Curve Widening