Roadside Safety Analysis Program Research Articles

Safety evaluation of design alternatives to separate freeway-opposing traffic

Objectives Different design alternatives used to separate freeway-opposing traffic may produce varying safety levels. State-of-the-art design guidelines have provided guidance in respect to a number of median features such as width, slope, and barrier installation. Guidance provided has been based on benefit-cost procedures; however, because project costs may significantly vary across different countries/jurisdictions, findings from benefit-cost procedures may be meaningless on a broader geographical scope. The objective of this study is to quantify the safety level of a number of design alternatives commonly used to separate opposing freeway traffic based solely on annualized crash costs. Methods The safety performance of 14 design alternatives were assessed using the Roadside Safety Analysis Program (RSAPv3). Results A test-level 3, low-tension cable barrier installed in the middle of a wide, hazard-free median was found to be the safest design alternative. Road sections containing: i) cable barriers were found to be safer than those containing w-beam guardrails, ii) low-tension cable barriers installed in the middle of the median were found to be safer than those containing high-tension cable barriers installed on one side of the median, regardless of median width, iii) wide, hazard-free medians were found to be safer than those containing WB guardrails, and iv) concrete barrier installations were found to be only safer than those containing unshielded medians, except when the median was 20 meters wide and hazard-free. Conclusions This paper not only provides evidence that some of the design alternatives often installed in real-world scenarios are not the safest, but it also discusses how some of these alternatives may not be the most cost-effective either. These findings make this study relevant and timely, as designers and policy/decision makers should always seek to maximize safety while optimizing the allocation of limited public funds.

Evaluation of Strategies to Mitigate Culvert-Involved Crashes

The objectives of this study were to assess the in-service safety performance of roadside culverts and evaluate the potential impacts of installing various safety treatments to mitigate the severity of culvert-involved crashes. Such crashes were identified using standard fields on police crash report forms, as well as through a review of pertinent keywords from the narrative section of these forms. These crashes were then linked to the nearest cross-drainage culvert, which was associated with the nearest road segment. A negative binomial regression model was then estimated to discern how the risk of culvert-involved crashes varied as a function of annual average daily traffic, speed limit, number of travel lanes, and culvert size and offset. The second stage of the analysis involved the use of the Roadside Safety Analysis Program to estimate the expected crash costs associated with various design contexts. A series of scenarios were evaluated, culminating in guidance as to the most cost-effective treatments for different combinations of roadway geometric and traffic characteristics. The results of this study provide an empirical model that can be used to predict the risk of culvert-involved crashes under various scenarios. The findings also suggest that the installation of safety grates on culvert openings provides a promising alternative for most of the cases where the culvert is located within the clear zone. In general, a guardrail is recommended when adverse conditions are present or when other treatments are not feasible at a specific location.

Optimal guardrail runout lengths for freeways

ABSTRACTGuardrails have commonly been installed to prevent errant vehicles from impacting roadside hazards. However, guardrail impacts have contributed to numerous serious injuries and fatalities. Plus, guardrails are generally impacted more often because they are installed closer to the edge of travel way and are much longer than the shielded hazard itself. Thus, to reduce the frequency of guardrail crashes, an optimized length should be determined. The American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide (RDG) has suggested guardrail runout lengths which are dependent on posted speed limit and traffic volumes. Crash data analyses and simulation using the recently-updated Roadside Safety Analysis Program (RSAPv3) was conducted to evaluate the guardrail length-of-need (LON) associated with the lowest crash cost (i.e., cost associated with injuries and property damage) and maximum cost-effectiveness for freeways. Crash data involving Kansas guardrail systems, which were compliant with recommendations provided in the 2006 AASHTO RDG and occurring on freeways with divided medians, were collected and analyzed. The frequency, rate, and risk of shielded hazard crashes were extremely low. RSAPv3 analyses indicated that there was an economic and safety benefit to reducing the installed LON as well as utilizing different runout lengths for left- and right-side departures for divided roadways.

Proposed Heavy Vehicle Encroachment Adjustment Factor

Encroachment probability models such as the Roadside Safety Analysis Program (RSAP) have traditionally assumed that heavy vehicles and passenger vehicles share the same encroachment characteristics. This assumption was reviewed in developing bridge railing selection guidelines in NCHRP 22-12(03), where an examination of a specific highway and a national sample of data indicated that trucks encroached at a different rate than passenger vehicles. This paper describes the development of a new vehicle-type encroachment adjustment factor (EAF). The results confirmed previous findings, but this analysis controlled for traffic volumes, highway type, percentage of heavy vehicles [i.e., percentage of trucks (PT)], and segment length. The result was a more robust model that was valid over a wider range of average annual daily traffic and PTs. The large data set included 635,464 segments of data from the states of Ohio and Washington. The proposed EAF was recommended for inclusion in RSAPv3. Ideally, encroachment data would be collected for heavy vehicles to determine the frequency of heavy vehicles encroaching onto the roadside and the trajectories heavy vehicles took during encroachment, but this process proved to be financially challenging. The study used crash data to carry out a comprehensive analysis of traffic volume, heavy vehicle mix, highway type, and segment length. A vehicle-type EAF was developed for divided and undivided roadways. The results provided some indication of how best to incorporate heavy vehicles in the encroachment probability model used in RSAP.

Proposed Horizontal Curve and Vertical Grade Encroachment Adjustment Factors

The probability of a vehicle encroaching onto the roadside is influenced by several geometric characteristics of the roadway. The horizontal curvature and vertical grade encroachment adjustment factors (EAFs) used in the third version of the Roadside Safety Analysis Program (RSAPv3) were adopted from adjustment factors introduced in 2003 in NCHRP Report 492 during the original development of RSAP. These adjustments were developed by interpreting earlier work on curves and grades. Given the weaknesses and limitations of the earlier study, it was suggested that a larger-scale study with larger sample size and more representative crash severities would be appropriate. This paper describes the development of new horizontal curve and vertical grade EAFs for rural areas. EAFs were developed from a cross-sectional model of crash data that controlled for geometric influences, traffic volumes, segment length, and highway type. The proposed EAFs are not identical to the current factors, which is not surprising, because the earlier study reviewed only 300 fatal crashes with fixed objects in 1975. This study reviewed the full range of crash severities and a range of run-off-road crash types and was conducted on much more recent crash data (2002–2010). Promisingly, however, EAFs are of the same order of magnitude and are broadly consistent with the earlier study.

Cost-Effective Safety Treatment of Trees on Low-Volume Rural Roads

Generally, trees are naturally occurring fixed objects that are found along many roadways and that potentially pose safety risks to errant motorists. Unfortunately, trees have been responsible for numerous fatal and serious injury crashes during run-off-road events. This study included an incremental benefit-to-cost (B-C) analysis that used the Roadside Safety Analysis Program to investigate the efficacy of safety treatment alternatives for trees on roadways with volumes of less than 500 vehicles per day (vpd) and speed limits of 55 mph (88.5 km/h) or greater. The study was based on a parametric analysis of site characteristics from a field survey in Kansas. It used four tree groupings, three tree diameters, and four lateral offsets from the roadway to configure 120 scenarios. Three safety treatment methods were considered: (a) a do-nothing option representing the baseline condition; (b) tree removal, with cost estimates coming from reliable sources; and (c) a crashworthy guardrail system. For various reasons, the guardrail system was no more cost-effective than the do-nothing or tree removal options. B-C ratios were used to recommend tree removal on the basis of several pertinent variables. In all cases, B-C ratios for tree removal were never less than 1.0, which indicated limited justification for keeping the trees. Tree removal was considered the safest and primary alternative when trees were far from other fixed obstacles. Because these guidelines are based solely on B-C analyses, the road designer or engineer is encouraged to use them as a foundation for making safety improvements but also to consider site-specific investigation and analysis.

Cost-Benefit Analysis of Crash Cushion Systems

Crash cushions vary in geometry and cost. In this study, crash cushions were categorized in three different categories: redirecting with repair costs greater than $1,000 (RGM), redirecting with repair costs less than $1,000 (RLM), and nonredirecting sacrificial (NRS). Typically, RGM systems are less expensive initially, but life-cycle costs are high. RLM systems typically reciprocate this trend. NRS crash cushions (e.g., sand barrels) are generally less expensive but require total replacement after a crash has occurred, which may be impractical at high-traffic volume locations. Due to limited funding, there is often a need to identify the most cost-effective crash cushion category for highway scenarios with different roadway, traffic, and roadside characteristics. This study was commissioned to determine benefit-cost ratios for each crash cushion category in a wide range of roadway and roadside characteristics using the probability-based encroachment tool, Roadside Safety Analysis Program. Only RGM and RLM systems were cost-effective for freeways and divided rural arterials, but all three categories competed against the unprotected condition on undivided rural arterials and local roads.

Correlation Between Crash Severity and Embankment Geometry

The severity of a roadside feature is often based on survey responses and has tended to emphasize extreme crash events, thereby overestimating the average severity of a particular feature. In this study, severity was related to embankment geometry by examining real-world accident data over a 7-year period. This was done by correlating the number of severe and fatal accidents to the exposure of particular slope geometries. Slope geometry was described by slope steepness and fill height, and its exposure was described by traffic volume and total unshielded mileage. Severity was adjusted for posted speed limits as well. The Roadside Safety Analysis Program (RSAP) was calibrated such that the distribution of severe injury and fatal accidents accurately reflected real-world data. Using this calibrated version of RSAP, the new severity indexes were studied and equations were created to correlate severity index to functional class, fill height, slope steepness, and posted speed limit. The local highway classification provided the highest severity, and the default severity used in RSAP was increased to accommodate this finding. Freeways, rural arterials, and urban arterials experienced reduced severity indexes relative to default values used in RSAP.

Benefits of Slope Flattening

The benefits of slope flattening were investigated by simulating accident costs with updated foreslope severities based on real-world accident data collected over a 7-year period in the State of Ohio. Functional classes considered were freeways, rural and urban arterials, and rural and urban local highways. Highways were modeled using the Roadside Safety Analysis Program (RSAP). Highway parameters considered in RSAP were slope steepness, roadway curvature, percent grade, longitudinal length, fill height, and lateral offset to the slope break point. Simulated accident costs were incorporated into a Microsoft Excel spreadsheet, where future users can specify installation costs, which tend to vary significantly from one location to another for slope flattening applications. Each functional class demonstrated slope flattening trends. On freeways and urban arterial highways, slopes should be no steeper than 1V:3H, and the benefit of flatter slopes was minimal. On rural arterial highways, the slope should be no steeper than 1V:4H, and the benefit of flatter slopes was also minimal. On local highways, the steepest slope should be 1V:3H, but the slope should be made as flat as possible because accident costs continued to decrease as the slope was flattened.

Method for Modeling Crash Severity with Observable Crash Data

This paper presents a new method to rank the severity of an impact with a roadside hazard that is based on observable crash data. This method has been incorporated into the third update of the Roadside Safety Analysis Program. The equivalent fatal crash cost ratio (EFCCR) is a dimensionless value that represents the severity of a crash on a scale of zero to unity, where zero represents no chance of injury and unity represents absolute certainty of a fatal injury. The method uses a census of police-reported data, which ideally covers a range of speed limits. The purpose is to determine the severity distribution of crashes in cases in which no events preceded the crashes with the hazard under evaluation and which did not result in a penetration or a rollover. The number of unreported crashes is estimated and added into the severity distribution with the assumption that the number represents crashes that led to property damage only. The average expected crash cost is then calculated and normalized to a reference speed of 65 mph so that it is directly comparable to EFCCR values calculated for other types of hazards. Unlike the earlier subjective severity index method, the new EFCCR method has its basis in observed crash data and uses a systematic approach to calculate crash severities that can be used in benefit–cost and other safety analyses.

Evaluating the Cost-Effectiveness of Roadside Culvert Treatments

Roadside cross-drainage culverts have been found to affect vehicle accident injury levels. As a result, highway designers have commonly used three safety treatments to protect errant motorists from striking culvert openings. These safety treatments have included: culvert extension, guardrail installation, and the application of safety grating. However, the identification of the most appropriate safety treatment for roadside culverts may be challenging; accident costs may dramatically change under different road and traffic characteristics. The purpose of this study was to estimate accident costs for a wide range of road and traffic scenarios and then define the safest treatment (i.e., treatment with lowest accident cost) for a variety of traffic, roadway, and roadside characteristics. Over 3,000 highway scenarios were modeled using the Roadside Safety Analysis Program (RSAP). This study showed that the selection of culvert safety treatments should be flexible when considering different road and traffic characteristics. The findings demonstrated that culvert extension and grating were found to produce the lowest accident costs for all highway scenarios that were modeled, and guardrail protection was not recommended for any of the scenarios. Therefore, it is believed that the expanded adoption of culvert extension and culvert grates can improve overall highway safety.

Guidelines for Implementation of Cable Median Barrier

A detailed examination of accidents in Kansas evaluated the need for a cable median barrier. Hard copies of all accident reports from Kansas controlled-access freeways from 2002 to 2006 were reviewed. A total of 525 cross-median events and 115 cross-median crashes were identified. Cross-median encroachment rates were linearly related to traffic volume and cross-median crash rates appeared to have a second-order relationship to volume. Cross-median crashes were much more frequent in winter months and the severity of wintertime crashes was lower. This finding indicates that median barrier warranting criteria may need to be adjusted to accommodate regional climate differences, especially for warranting guidelines based on cross-median crash rates. A relationship was found between cross-median crash rate and traffic volume for Kansas freeways with median widths of 60 ft (18.3 m). This relationship was combined with encroachment rate and lateral extent of encroachment data from the Roadside Safety Analysis Program to develop general guidelines on the use of cable median barriers along Kansas freeways.

Placement Guidance for Temporary Concrete Barriers in Construction Work Zones

An important design decision in construction work zones concerns the use and placement of temporary barriers. The temporary concrete safety shaped barrier is the option preferred by most state transportation agencies. No consensus on specific warrants or nationally recognized guidance exists. Of 32 state departments of transportation responding to a recent survey, 24 ranked having or improving work zone guidance on “traffic barriers and roadside design” as “most important/critical.” The objective of this paper is to investigate and evaluate the information, tools, and procedures available to aid individual designers and agencies in developing guidance for the use of traffic barriers in construction work zones. Principles of roadside barrier use for permanent roadways will be covered. Also covered will be benefit–cost analysis and adaptation of permanent roadway procedures and programs, specifically the Roadside Safety Analysis Program (RSAP), for the purpose of developing work zone barrier guidance. Results show that the modeling of specific work zone scenarios and development of generalized barrier placement guidance for practitioners based on cost-effective principles are both feasible. This type of analysis can be done by adapting algorithms in RSAP intended for permanent roadway analysis to construction work zones. General guidelines similar to the median barrier warrants for permanent roadways in the Roadside Design Guide were developed for construction work zones. In addition, it was concluded that current encroachment models and crash severity indexes may not be well suited for permanent roadways or construction work zones.

Placement Guidance for Temporary Concrete Barriers in Construction Work Zones: Developed with Roadside Safety Analysis

An important decision in construction work zones concerns the use and placement of temporary barriers. The temporary concrete safety shaped barrier is the option preferred by state transportation agencies. No consensus on specific warrants or nationally recognized guidance exists. Of 32 state departments of transportation responding to a recent survey, 24 ranked having or improving work zone guidance on barriers and roadside design as most important/critical. The objective of this paper is to investigate and evaluate the information, tools, and procedures available to aid individual designers and agencies in developing guidance for the use of traffic barriers in construction work zones. Principles of roadside barrier use for permanent roadways will be covered. Also covered will be benefit-cost analysis and adaptation of permanent roadway procedures and programs, specifically the Roadside Safety Analysis Program (RSAP), for the purpose of developing work zone barrier guidance. Resul...

Roadside Safety Analysis Program as a Tool for Economic Evaluation of Roadside Safety Projects

Highway agencies are continually called upon to make decisions about roadside safety projects by considering their relative benefits and costs. ROADSIDE software is a tool developed in the late 1980s for costeffectiveness analysis of safety features and has been used by different highway agencies. As a part of the continuing effort to improve such costeffectiveness procedures, the Roadside Safety Analysis Program (RSAP) has recently been developed at the Texas Transportation Institute. RSAP uses a simulation technique to replicate single-vehicle accidents on a highway facility and can test the effectiveness of the various countermeasures that are used to reduce the severities of these accidents. The application of RSAP to actual case study sites and the outcome of a study recently conducted at Wayne State University are described. Various median treatments on freeways were tested, and their respective benefit-cost ratios, as estimated by RSAP output, were examined. A set of sensitivity analyses of the RSAP output to various input parameters was also conducted. It is found that RSAP is user-friendly software and is fully operational on a personal computer with a Pentium processor in the Windows 95 environment. RSAP provides a summary of the accidents predicted for each scenario tested, and those are segregated by each feature coded along with its respective severity and cost. The economic analysis procedure is based on strong analytic principles, and the results are sensitive to accident cost, cost of construction, and interest rate. The software output is generally sensitive to the input parameters, and the trends appear to be logical. Additional research to calibrate the accident prediction model with historic accident data at the project site will be highly desirable.

Roadside Safety Analysis Program: A Cost-Effectiveness Analysis Procedure

Brief descriptions are provided of a new cost-effectiveness analysis program, known as the Roadside Safety Analysis Program (RSAP), which was developed under NCHRP Project 22-9. RSAP is an improvement over existing cost-effectiveness analysis procedures for evaluation of roadside safety improvements, such as the procedures in the 1977 AASHTO barrier guide and the ROADSIDE program. RSAP improves on many of the algorithms in the procedures and provides a user-friendly interface to facilitate use. The program has undergone extensive testing and validation, including evaluation by an independent reviewer. It is anticipated that RSAP will be available to the public through the McTrans Center at the University of Florida.