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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Page 2

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1 Design guidelines are presented for providing the horizontal sightline offset needed to provide the design stopping sight distance (DSSD) specified in AASHTO’s A Policy on Geometric Design of Highways and Streets (known as the Green Book). The guidelines include procedures to assess horizontal curves with sight obstructions on the inside of the curve that limit the horizontal sightline offset. The distance between the driver’s line of sight along the roadway ahead on a horizontal curve and a sight obstruction on the inside of the curve is known as the horizontal sightline offset (HSO). The design criteria for the horizontal component of stopping sight distance (SSD) in the Green Book are based on the maximum sightline offset that may be needed at any point along a curve with a given radius. The Green Book does not fully explain that sightline offset less than the maximum is needed toward the ends of horizontal curves longer than the DSSD and at any point along curves shorter than the DSSD. In addition, the Green Book does not show that some sightline offset is needed even beyond the ends of a horizontal curve. The relationship between SSD and the frequency and severity of crashes has been difficult to quantify because the role of SSD in reducing crashes is highly situational. A sight-distance-related crash can only occur where a sight distance limitation is present, a vehicle is approaching the sight distance limitation, and an object that the driver of the approaching vehicle needs to see is present in the sight-restricted area. Objects that the approaching driver needs to see are much more likely at some locations than at others. The crash investigation results indicate that sight-distance-related crashes are so infre- quent and so difficult to identify definitively that it is not feasible to develop predictive models for crashes related to SSD or to quantify the crash reduction effectiveness of sight distance improvements. Therefore, several tools, including a reliability analysis approach, are presented here to assist designers in assessing horizontal sight distance issues. A series of equations is provided to assist planners and designers in identifying the shape and extent of the roadside area that should be clear of sight obstructions so that the applicable DSSD is available. The equations can potentially be used in computer- aided design and drafting (CADD) systems so that the area that should be free of sight obstructions is displayed visually to designers. A benefit-cost equation allows planners and designers to estimate an upper limit on the project implementation cost that would be justified to remove or mitigate horizontal sight obstructions. A reliability analysis model was developed to assist designers in assessing existing horizontal curves and horizontal curves under design for new construction to prioritize S U M M A R Y Design Guidelines for Horizontal Sightline Offsets

2 Design Guidelines for Horizontal Sightline Offsets the removal or mitigation of horizontal sight obstructions. The reliability analysis model computes the longitudinal distribution or profile of available stopping sight distance (ASSD) along the site and estimates the number and percentage of approaching vehicles per year in each travel lane that may encounter stopped vehicles in the sight-restricted area due to crashes or congestion-related queues. The reliability analysis model has been incorporated in a spreadsheet tool for application by planners and designers. Procedures for assessing the need for removal or mitigation of horizontal sight obstruc- tions are presented, together with lessons learned from case studies of seven sites with existing horizontal sight restrictions. Mitigation strategies that may be applied where a decision is reached not to remove a horizontal sight obstruction are presented. The practicality of mitigation strategies depends on the cost of the applicable mitigation strategies and the priority assigned to the site based on the results of the reliability analysis model. The situations in which design exception documentation should be prepared are also identified.

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The distance between the driver’s line of sight along the roadway ahead on a horizontal curve and a sight obstruction on the inside of the curve is known as the horizontal sightline offset (HSO). Highway agencies can use NCHRP Research Report 910: Design Guidelines for Horizontal Sightline Offsets as guidance to address the types of sight distance restrictions that are most likely to be encountered on specific roadway types.

The relationship between stopping sight distance (SSD) and the frequency and severity of crashes has been difficult to quantify because the role of SSD in reducing crashes is highly situational. The design criteria for the horizontal component of SSD in what is known as AASHTO's Green Book are based on the maximum sightline offset that may be needed at any point along a curve with a given radius, which doesn't cover all possible situations.

Designers compensate for the limitations on driver sight distance in various ways, including: accepting shorter sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs; as a result, many highway agencies have used the design exception process to address the trade-offs for sight distance in such situations.

This project conducted research to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered. The project includes a tool (an Excel spreadsheet) that may be used to calculate sight distance.

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