National Academies Press: OpenBook

Design Guidelines for Horizontal Sightline Offsets (2019)

Chapter: Chapter 3 - Relationship of Sight Distance to Crash Frequency and Severity

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Suggested Citation:"Chapter 3 - Relationship of Sight Distance to Crash Frequency and Severity." 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 21
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Suggested Citation:"Chapter 3 - Relationship of Sight Distance to Crash Frequency and Severity." 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 22

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21 Only limited research has examined the relationship of sight distance to crash frequency and severity and no studies have developed any generally applicable CMFs. Glennon (1987) reviewed seven published studies in which SSD was considered one of several factors that might affect crash rates. These studies included work by Cirillo et al. (1969), Foody and Long (1974), Gupta and Jain (1973), Hills (1977), Raff (1953), Schoppert (1957), and Sparks (1968). Each of these studies used some form of either multivariate analysis or a sufficiency rating scheme to address SSD effects. Glennon concluded that none of these studies provided any reliable method of determining the effects of SSD on crashes. Olson et al. (1984) evaluated the crash history of 10 pairs of crest vertical curve sites. One site of each pair was a crest vertical curve with limited SSD (118 to 308 ft), while the other was a similar nearby crest with adequate SSD (greater than 700 ft). Olson et al. (1984) found that in seven of the 10 pairs, the limited SSD site had more crashes than the adequate SSD site. In one of the pairs, the adequate SSD site had more crashes. For two of the 10 pairs of sites, there was no difference in crashes between the paired sites. For the 10 sites as a whole, the sites with limited SSD experienced 50 percent more crashes than the sites with adequate SSD. This study provides some evidence for an effect of SSD on crashes, but the study has flaws that limit its applicability. The Olson et al. (1984) crash study did not compensate for regression-to-the-mean bias and did not document what roadway features, if any, were hidden by the sight distance limitation. Limited research has also addressed crash relationships for sight distance types other than SSD. For example, recent research by Himes et al. (2016) has established that intersections with greater intersection sight distance (ISD) have fewer crashes, including fewer fatal-and-injury crashes, than intersections with less ISD. Results from Himes et al. (2016) indicate that ISD has a greater effect on crash frequency at higher traffic volumes than at lower traffic volumes. The lack of crash reduction effectiveness measures for SSD is puzzling, given the emphasis placed on SSD in design. Generations of highway engineers have been taught that SSD must be provided at all points along the roadway alignment. Yet, despite this perceived importance, there are no definitive CMFs that quantify the effect of SSD on crashes in either the AASHTO Highway Safety Manual (AASHTO 2010; AASHTO 2014) or on the FHWA CMF Clearinghouse website (www.cmfclearinghouse.org). The likely reason that the safety effects of SSD have not been successfully quantified is that these effects are highly situational, meaning that limited SSD is far more likely to result in a collision at some locations than at others. Recent research in NCHRP Report 783 (Harwood et al. 2014) found that, at crest vertical curves with limited SSD on rural two-lane highways, crash frequencies were high at locations where intersections, driveways, or horizontal curves were hidden from the approaching driver’s view by the sight restriction. However, where no hidden features were present, there was much lower crash experience, even though the SSD was limited. On other highway types, such as divided highways, C H A P T E R 3 Relationship of Sight Distance to Crash Frequency and Severity

22 Design Guidelines for Horizontal Sightline Offsets freeways, or urban arterials, a variety of additional hidden features, such as ramp terminals or pedestrian crossings, may also be critical if located in a sight-restricted area. And, on congested highways, there is a possibility of a standing queue being present in a sight-restricted area during specific time periods. This research demonstrates for crest vertical curves that correcting or mitigating limited SSD may be much more critical in some highway situations than in others; the same principle is likely to apply to horizontal sight restrictions. Considering the findings of NCHRP Report 783, it is reasonable that Himes at al. (2016) found an effect of ISD on crashes— since an intersection is present by definition at locations where ISD is limited—while previous research on sites with limited SSD, but not necessarily with critical features present in the sight-limited area, found no effects or inconsistent effects. Research by Potts et al. (2018) studied curves with limited horizontal sight distance and found that sight-distance-related crashes were difficult to identify explicitly from either electronic crash data or hard-copy police crash reports. However, it was evident that sight-distance-related crashes are rare events. Sight-distance-related crashes were found to be so infrequent that there was not sufficient data to develop formal CMFs for removal of horizontal sight obstructions. In fact, given the varying criticality of SSD, depending on the presence or absence of hidden features in the sight-restricted area, it is unlikely that generally applicable CMFs for SSD improvements can be developed. Given the lack of CMFs for sight distance improvements, alternative approaches are needed to prioritize locations for improvement. This guide presents two tools that can assist highway agencies in analyzing and setting priorities for horizontal sight distance improvements: • A benefit-cost analysis procedure that uses a user-supplied estimate of the maximum crash reduction likely to result from a project to estimate the maximum implementation cost that should be invested in such a project (see Chapter 4). • A reliability analysis model and accompanying spreadsheet-based tool that can quantify the extent of a horizontal sight distance restriction and can estimate the number of approaching vehicles per year on a particular curve that are likely to encounter a stopped vehicle in the sight-restricted area (see Chapter 5). Chapter 6 presents a step-by-step procedure for using these tools to assess whether horizontal sight restrictions on particular horizontal curves should be removed or mitigated. Available mitigation strategies are reviewed in Chapter 7.

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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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