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                        Maintenance of Drainage Features for Safety


            
            In This Section
        Local and Rural Road Safety Program
                Training, Tools, Guidance and Countermeasures for Locals Practitioners
                Crash Facts
                
                Appendix A: CLEAR ZONE DESCRIPTION
The concept of clear zone is an approach to minimize the number and severity of crashes involving vehicles running off the road or encountering slopes that may lead to instability and overturning. Simply stated, it is a traversable area that starts at the edge of the traffic lane and extends laterally a sufficient distance to allow a driver to stop or return to the road before encountering a hazard or overturning. The traversable area would be considered safe, if there were no fixed objects, unless they are breakaway, and if the roadside geometry (either the fore slope, back slope, or ditch) was flat enough that a vehicle could safely traverse the area without tipping and rolling over. Roadside safety features include breakaway sign and light posts, and traversable drainage structures. Curbs are not considered a roadside safety feature since they can be easily mounted by errant vehicles; hence, their presence does not alter how clear zone is measured.
"Diagram. This diagram shows the parts of a clear zone. From left to right they are: back slope, drainage ditch, side slope, hinge point, shoulder, cross slope. The cross slope makes up the traveled way. The cross slope plus the shoulder makes up the roadway. The shoulder, hinge point, and side slope make up the clear zone. The Hinge point is defined as the point where the slope rate changes. The clear zone is defined as the area along the side of the traveled way including the shoulder that is available for recovery of an errant vehicle."***Clear zone illustration***

A safe traversable slope can be either a recoverable slope or a non-recoverable slope with a clear run-out area at the bottom. A recoverable slope is a slope on which a motorist may, to a greater or lesser extent, retain or regain control of a vehicle and recover or stop. Slopes 1:4 (Vertical:Horizontal) or flatter are generally considered recoverable. A non-recoverable, traversable slope is a slope which is considered traversable but on which an errant vehicle will continue to the bottom. Embankment slopes from 1:3 and 1:4 may be considered traversable but non-recoverable if they are smooth and free of fixed objects. A clear run-out area is the flatter area at the toe of a non-recoverable slope available for safe use by an errant vehicle. Slopes steeper than 1:3 are not considered traversable and should not be found in the clear zone.

The objective of roadside safety is to provide and maintain as much clear zone as practical. The design clear zone is the minimum width to be provided on a project and is dependent upon speeds, the roadside geometry, and traffic volumes. Further details on clear zone can be found in the *Roadside Design Guide*.
      
  Book traversal links for Appendix A: CLEAR ZONE DESCRIPTION
      ‹ VI. REFERENCES
                    Up
                    Appendix B: THREE TYPICAL WORK ZONE LAYOUTS FROM MUTCD ›
                
                Book:Maintenance of Drainage Features for Safety
              I. Introduction
                II: HOW STORM RUN-OFF AFFECTS ROADWAY SAFETY
                III: RECOGNIZING DRAINAGE PROBLEMS
                IV: CORRECTING UNSAFE DRAINAGE FEATURES
                V: WORKER SAFETY
                VI. REFERENCES
                Appendix A: CLEAR ZONE DESCRIPTION
                Appendix B: THREE TYPICAL WORK ZONE LAYOUTS FROM MUTCD U.S. DEPARTMENT OF TRANSPORTATION

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