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Horizontal Sightline Offset (HSO) Criteria for Freeways, Ramps, and Other Similar Roadways


When horizontal curves are bound by barrier walls or other potential sight line impediments (i.e., retaining walls, parapet walls, abutments, piers, columns, and sound barrier), especially in high speed roadways and system ramp connections, designers compensate for the impact to driver sight distance in various ways including lowering design speed, increasing shoulder width, providing additional signage, or other mitigation strategies. There are drawbacks to each of these mitigation strategies. Lowering the design speed can result in decreased superelevation without any other visual cues for the driver to lower their speed. Another adverse result can lead to extra wide shoulders (ranging from 16 feet to 20+feet) along the inside of the horizontal curve when the barrier/visual impediment is located on the same side. Providing extra wide shoulder increases costs (especially if on structure), may increase Right of Way (ROW) needs, and may encourage erratic behavior from drivers who believe the additional area was intended for parking or as an additional driving lane (this can lead to operational and safety implications). In ahddition, these concerns may prevent agencies from installing High Occupancy Lanes (HOV) lanes to manage capacity problems. Because of these conflicting factors, many agencies have used the design exception processes to address the tradeoffs for sight distance in such situations. This research would examine these situations to determine whether criteria for horizontal sightline offset could be developed to better address these conflicting factors. The research should evaluate this condition and determine what criteria will provide an acceptable design. Research should examine existing research, current design criteria, performance and safety of existing locations with such geometrics, identify mitigation techniques and determine whether the design criteria should be revised. When studied, the researchers should take into consideration the impact of the vertical alignment in addition to the horizontal offset to the wall or impediment. The researchers should also examine the geometrics under various traffic conditions. Dense traffic may be another impacting factor in determining the Horizontal Sightline Offset (HSO) criteria. In the case of barrier wall and parapets, glare screens may also be a desirable design element.


The objective of this research is to provide horizontal offset design criteria for curved alignments adjacent to barrier and similar types of impediments that may impact the driver’s line of sight. The research would be helpful to the transportation design professional and facility owners. The findings may lead to modification of the design criteria and could reduce costs, reduce ROW needs, and eliminate some future design exceptions. The results of the project should be developed in a manner that allows it to be directly used within future updates to the AASHTO Green Book.

Related Research:

Freeway and Interchange Geometric Design Handbook. Institute of Transportation Engineers, Joel Leisch (ed.), Washington, DC, 2005.

NCHRP 08-88-Guidelines for Ramp and Interchange Spacing. Kittelson & Associates, Inc. Jan. 2010.

Wang, Zhongren. “The Sight Distance Issues with Retrofitted Single-Lane HOV Facilities”, International Journal of Transportation Science and Technology, V2 N2, 2013.

Chiu, Liu. “Exact Sight Distance Determination on Compound Vertical and Horizontal Curves in the Presence of Road Barriers”, International Journal of Transportation Science and Technology, V2 N2, 2013.

Mauga, Timur; Ghanma, Mwafag; and Ahmed, Kamran “Roadside Clearance Limit on Horizontal Curves with Transition Arcs: Sites with Circular Arcs Shorter than Sight Distance” TRB 92nd Annual Meeting Compendium of Papers 13-3113, 2013.

Sarhan, Mohamed and Hassan, Yasser. “Consideration of Sight Distance in Placement of Concrete Barriers on Horizontal Curves of Roads”, Operational Effects of Geometrics and Access Management 2012. Report 12-0612, 2012.

Arndt, Owen; Cox, Ricky; Sandra, Lennie;, and Whitehead, Mike. “Provision of Sight Distance around Concrete Barriers and Structures on Freeways and Interchanges” 4th International Symposium on Highway Geometric Design, June 2-5, 2010 Valencia Spain. TRB. 2010


Some of the tasks that could be completed in this project proposal include:

· Completion of a comprehensive literature review and summary of current practice. This should include both international domestic resources and practices. This review should also identify locations with reduced HSO and full HSO so data can be collected from these locations. To identify these locations, an owner/agency survey will likely be needed.

· Develop and perform owner/agency survey.

· Identify which design exceptions are being used and under what conditions/criteria.

· Identify the geometric conditions for each location in the horizontal, vertical, and cross section. Data collected should include height of barrier, parapet, or top of glare screen when applicable.

· Determine which mitigation strategies have been used to help offset the design criteria that utilized a horizontal offset design exception.

· Collect safety data for the various geometric options and, if a design exception was approved, include any such mitigation strategies

· Evaluate the collected data

· Provide recommendations for horizontal sight offset from driver to object. Recommendations should include horizontal offset distances with any recommended mitigation strategies.

· Propose new text based on the results of the research project for the next edition of the AASHTO Green Book.


This research need has been identified as a high priority by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their July 2013 combined meeting. The research is needed to provide enhanced guidance that will directly impact future horizontal sight offset design practices for use nationally in the design of interchanges and other high speed turning roadways. The potential payoff is the selection of more cost efficient, safer, and contextually appropriate designs.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
Research Priority:High
RNS Developer:James Rosenow, Zhongren Wang, Jim Brewer, Deanna Maifield, David McDonald, Annette Riley, Jerry Champa, Mike Fugett, Mark Doctor
Source Info:Problem statement developed as a result of the Safety Effects of Geometric Design Decisions Workshop at the 2013 mid-year meeting of TRB Committees AFB10 (Geometric Design) and AHB65 (Operational Effects of Geometrics), in conjunction with the AASHTO Technical Committee on Geometric Design. Problem Statement # 2015-G-24 on the NCHRP/AASHTO SCOR list.
Date Posted:02/07/2014
Date Modified:02/10/2014
Index Terms:Highway design, Highway curves, Line of sight, Right of way (Traffic), Traffic density, Ramps (Interchanges), Freeways,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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