Clear Zone and Horizontal Clearance
Frequently Asked Questions
- What is the definition of clear zone?
The Roadside Design Guide defines a clear zone as the total roadside border area, starting at the edge of the traveled way, available for safe use by errant vehicles. This area may consist of a shoulder, a recoverable slope, a non-recoverable slope, and/or a clear run-out area. The desired minimum width is dependent upon traffic volumes and speeds and on the roadside geometry. Simply stated, it is an unobstructed, relatively flat area beyond the edge of the traveled way that allows a driver to stop safely or regain control of a vehicle that leaves the traveled way.
A recoverable slope is a slope on which a motorist may, to a greater or lesser extent, retain or regain control of a vehicle by slowing or stopping. Slopes flatter than 1V:4H are generally considered recoverable. A non-recoverable slope is a slope which is considered traversable but on which an errant vehicle will continue to the bottom. Embankment slopes between 1V:3H and 1V:4H may be considered traversable but non-recoverable if they are smooth and free of fixed objects. A clear run-out area is the area at the toe of a non-recoverable slope available for safe use by an errant vehicle. Slopes steeper than 1V:3H are not considered traversable and are not considered part of the clear zone.
- Where can I find information on clear zone dimensions?
The current edition of the AASHTO Roadside Design Guide presents information on the latest state-of-the-practice in roadside safety. It presents procedures to determine a recommended minimum clear zone on tangent sections of roadway with variable side slopes and adjustments for horizontal curvature.
The AASHTO A Policy on Geometric Design of Highways and Streets (Green Book) enumerates a clear zone value for two functional classes of highway. For local roads and streets, a minimum clear zone of 7 to 10 feet is considered desirable on sections without curb. In the discussion on collectors without curbs, a 10-foot minimum clear zone is recommended. The general discussion on Cross-section Elements also indicates a clear zone of 10 ft. for low-speed rural collectors and rural local roads should be provided.
- What is the definition of horizontal clearance?
Horizontal clearance is the lateral offset distance from the edge of the traveled way, shoulder or other designated point to a vertical roadside element. These dimensional values are not calculated, and are not intended to constitute a clear zone. They are intended to provide a roadside environment that is not likely to have an adverse affect on motorists' using the roadway. These lateral offsets provide clearance for mirrors on trucks and buses that are in the extreme right lane of a facility and for opening curbside doors of parked vehicles, as two examples.
- What are some examples of roadside elements requiring horizontal clearance?
Curbs, walls, barriers, piers, sign and signal supports, mature trees, landscaping items, and power poles are primary examples of the type of features that can affect a driver's speed or lane position if located too close to the roadway edge. Other specific examples can be found in the Cross Section Elements, Local Roads and Streets, Collector Roads and Streets, Rural and Urban Arterials, Freeways, and Intersections chapters of the Green Book.
The AASHTO A Policy on Design Standards - Interstate System also contains a discussion on horizontal clearance in the section Horizontal Clearance to Obstructions.
- Is clear zone a controlling criterion?
No. Controlling criteria are 13 items or elements in the NHS design standards that require a formal design exception when the adopted minimum value is not met on a project. The list of controlling criteria was developed to insure that deviations below the adopted value for a critical element were adequately considered in design of a project. When the original list was developed in 1985, clear zone was considered to be synonymous with horizontal clearance. Subsequently, in 1990, following adoption of the Roadside Design Guide, it was decided that clear zone width would no longer be considered as an element requiring a formal design exception. In the rulemaking to adopt the Roadside Design Guide, it was determined that clear zone width should not be defined by a fixed, nationally applicable value. The various numbers in the guide associated with clear zone are not considered as exact but as ranges of values within which judgment should be exercised in making design decisions. Objects or terrain features that fall within the appropriate clear zone are typically shielded so a design exception is not needed. The FHWA believes that a consistent design approach, guided by past crash history and a cost-effectiveness analysis is the most responsible method to determine appropriate clear zone width.
While clear zone is not a controlling criterion for purposes of applying the Green Book to the National Highway System, an exception to a clear zone for a project does need to be noted, approved and documented in the same manner as exceptions to other non-controlling criteria when the established value is not met. The documentation may be included in project notes of meetings or other appropriate means.
- Where is the appropriate location for above ground utility structures?
FHWA policy is that utility facilities should be located as close to the right-of-way line as feasible. The Green Book, AASHTO Highway Safety Design and Operations Guide, 1997, (Yellow Book) and the AASHTO A Guide for Accommodating Utilities within Highway Right-of-way, all state that utilities should be located as close to the right-of-way line as feasible. The Yellow Book, recognizing that crashes are overrepresented on urban arterials and collectors, says this means as far as practical behind the face of outer curbs and where feasible, behind the sidewalks.
It is not always feasible to relocate all poles within project limits. Critical locations should be considered for improvement, such as those dictated by crash experience or in potential crash locations, such as within horizontal curves. Where poles cannot be relocated from critical locations, mitigation such as breakaway or shielding should be considered. Poles should not be installed in a location that could act as a funnel directing an errant vehicle into an obstacle (for example a roadside drainage ditch, that would also disrupt the hydraulics). Locating a pole as far as feasible from the traveled way improves sight lines and visibility, providing a safer roadside.
- What clear zone needs to be maintained behind a curb?
The difference between a "clear zone" and horizontal clearance or "operational offset" has been a topic of much confusion. When the Green Book and the Roadside Design Guide were last updated, the AASHTO committees coordinated to dispel the misunderstanding that 2 feet (actually, 18 inches) behind a curb constituted a clear zone. Since curbs are now generally recognized as having no significant containment or redirection capability, clear zone should be based on traffic volumes and speeds, both without and with a curb. Realizing that there are still contradictory passages in various AASHTO documents, the Technical Committee on Roadside Safety has initiated a short-term project to identify all such inconsistencies and to recommend appropriate language corrections. This effort is underway. The fourth paragraph under Section 3.4.1 Curbs in the 2002 Roadside Design Guide correctly defines AASHTO's "position".
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