AASHTO Roadside Design Guide 4th Edition The purpose of this memo is three-fold: reiterate the status of the RDG for FHWA, summarize significant changes in the RDG 4th edition, and add Frequently Asked Questions to our website. Coordination of Vertical Clearance Design Exceptions on the Interstate System.
Clear ZonesBy creating Clear Zones, roadway agencies can increase the likelihood that a roadway departure results in a safe recovery rather than a crash, and mitigate the severity of crashes that do occur.A Clear Zone is an unobstructed, traversable roadside area that allows a driver to stop safely, or regain control of a vehicle that has left the roadway. The width of the clear zone should be based on risk (also called exposure). Key factors in assessing risk include traffic volumes, speeds, and slopes. Clear roadsides consider both fixed objects and terrain that may cause vehicles to rollover.Horizontal ‘clearance’ must not be confused with ‘clear zone’.
The minimum 18 inch horizontal clearance to objects behind curbs that is specified in the AASHTO Green Book is a minimum standard offset that allows for normal traffic operations. Because curbs do not deter errant vehicles from leaving the traveled way, the minimum horizontal clearance does not provide a clear zone sufficient to accommodate errant vehicles.
The recommended clear zones in the AASHTO Roadside Design Guide (RDG) are based on the design speed of the facility and the slope of the roadside and are not affected by the presence of curbs. It is recognized that providing a clear zone as recommended in the RDG may not be practical in low speed curbed facilities because of right-of-way constraints and other realities of the built environment and a design exception is not required in these cases. However, the minimum 18 inch horizontal clearance to vertical obstructions must still be met unless a design exception is approved.Clear Zones.This document provides guidance to help highway agencies develop their own standards and policies for determining the widths of clear zones along roadways based on speed, traffic volume, roadside slope and curvature. The recommended clear zone ranges are based on a width of 30 to 32 feet for flat, level terrain adjacent to a straight section of a 60mph highway with an average daily traffic of 6000 vehicles. For steeper slopes on a 70 mph roadway the clear zone range increases to 38 to 46 feet, and on a low speed, low volume roadway the clear zone range drops to 7 to 10 feet.
For horizontal curves the clear zone can be increased by up to 50 percent from these figures.The MUTCD requires that sign supports within the clear zone be made breakaway or shielded by a barrier. All new sign installations shall meet these criteria. Existing sign supports located on highways posted at 50 mph or greater shall meet this criterion by January 10, 2013. On roads posted at speeds 45 mph or lower, the breakaway criterion may be met when upgrading sign retroreflectivity.This page provides additional clarification on the design of clear zones and the distinction between clear zone and horizontal clearance.Trees and Landscaping Considerations.This report provides agencies with examples of successful–immediately deployable–tree and pole practices. These practices range from complex, multi-million dollar contract solutions to in-house practices.DVD: Highway Safety and Trees: The Delicate BalanceTrees are the single most commonly struck objects in roadside crashes.
This 12-minute DVD produced by the FHWA encourages communities to work with roadway agencies to improve safety, while minimizing environmental impacts. The DVD was designed for use at State and local highway agency meetings, public hearings, and town meetings. To obtain copies, e-mail, and request publication number FHWA-SA-06-13.This tri-fold brochure presents an overview of issues related to trees and highway safety, and tells where to find more information.A Guide for Addressing Collisions with Trees in Hazardous Locations , 981 KBA Guide for Addressing Collisions with Trees in Hazardous Locations is Volume 3 of NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan.
Geometric DesignTitle 23 USC 109 provides that design standards for projects on the (NHS) must be approved by the Secretary of the U.S. Department of Transportation in cooperation with the State highway departments.
The Secretary has delegated this authority to the Federal Highway Administrator.The State highway departments, working through the American Association of State Highway and Transportation Officials develop design standards through a series of committees and task forces. FHWA contributes to the development of the design standards through membership on these working units, sponsoring and participating in research efforts, and many other initiatives. Following development of the design standards, FHWA uses a formal rulemaking process to adopt those it considers suitable for application on the NHS.