Frank Gross

Safety Effectiveness of Lane and Shoulder Width Combinations on Rural, Two-Lane, Undivided Roads

There is a need to evaluate low-cost safety strategies that states may implement as part of their Strategic Highway Safety Plan. FHWA organized a pooled fund study of 26 states to evaluate several low-cost safety strategies, including the reallocation of total paved width. This study identifies whether it is safer to increase lane width or increase shoulder width given a fixed total width. Geometric, traffic, and crash data were obtained for more than 52,000 mi of roadway in Pennsylvania and Washington State. A case-control approach was applied to evaluate the safety effectiveness of various lane–shoulder configurations. There was a general reduction in the odds ratio as total paved width, lane width, and shoulder width increased individually; this is consistent with previous research. However, the primary research objective was to estimate the safety effectiveness of reallocating a fixed total paved width. Individual state results did not indicate a clear trade-off between lane and shoulder width for a fixed total width. Supplementing the results of this study with previous research, crash modification factors (CMFs) are provided for several lane–shoulder combinations. The selected values present a more apparent trade-off, indicating a slight benefit to increasing lane width for a fixed total width. Importantly, the results differ from other studies that developed CMFs without considering the interaction between lane and shoulder width, including those studies currently referenced in the Highway Safety Manual. This raises the question of whether CMFs should reflect the interaction between lane and shoulder width.

Safety effectiveness of converting signalized intersections to roundabouts

Roundabouts may be new builds but often are conversions from existing intersections. When contemplating the later, there is a need to estimate the safety effects of conversions. Several studies have estimated large reductions in crashes and severity; however, these results pertain mainly to conversions from unsignalized intersections. Results for conversions from signalized intersections have been less conclusive or consistent and tend to be somewhat dated. The objective of this study was to fill this void by estimating the safety effectiveness of converting signalized intersections to roundabouts. Several states helped to identify signalized intersections that were converted to roundabouts in the recent past. In total, 28 conversions were identified in the United States. The empirical Bayes (EB) method was employed in an observational before-after study to estimate the safety effects. Data from select states were also used in a cross-sectional analysis to investigate the compatibility of results from cross-sectional and before-after studies. The EB results indicated a safety benefit for converting signalized intersections to roundabouts. There were reductions in both total and injury crashes, with a larger benefit for injury crashes. Further analysis indicated that the safety benefit of roundabouts for total crashes decreased as traffic volumes increase, a result that suggests the need for the development of a crash modification function, a task for which more data would be required. The safety benefit for injury crashes was sustained across all traffic volumes. Both trends were supported by the cross-sectional analysis. Based on the analysis, it appears that roundabouts have the potential to significantly reduce crashes and severity at signalized intersections. A key aspect of the study was the estimation of the standard deviation of the distribution of the CMF in addition to the conventionally estimated standard error of the mean CMF value. For some CMFs, especially the CMFs for total crashes, the standard deviation of the distribution was larger than the standard error of the mean value of the CMF, indicating substantial variation in the treatment effect across sites.

Safety Effectiveness of Selected Treatments at Urban Signalized Intersections

This study conducted a before–after evaluation by means of the empirical Bayes methodology for four types of treatments at signalized intersections with data from Winston-Salem, North Carolina. The results indicated that changing to protected left-turn phasing from permissive or permissive–protected phasing could lead to a virtual elimination of left-turn crashes but other crashes, which were likely to be less severe, could increase. Conversion from nighttime flashback to regular phasing seemed effective in reducing nighttime crashes. Replacing 8-in. signal heads with 12-in. heads seemed effective in reducing right-angle crashes, but this measure could increase other, less-severe crashes. Adding another red-signal lens to an existing one or changing from permissive to permissive–protected left-turn phasing did not seem particularly effective in reducing crashes, but these results were not definitive because they were based on a limited number of sites. Further research using data from other jurisdictions is needed, so that more definitive conclusions can be made about the safety effectiveness of these treatments.

Analysis of Naturalistic Driving Data: Prospective View on Methodological Paradigms

Recently completed naturalistic (i.e., unobtrusive) driving studies provide safety researchers with an unprecedented opportunity to study and analyze the occurrence of crashes and a range of near-crash events. Rather than focus on the details of the events immediately before the crash, this study seeks to identify methodological paradigms that can be used to answer questions long of interest to safety researchers. In particular, an attempt is made to shed some light on the four important components of methodological paradigms for naturalistic driving analysis: surrogates, evaluative aspects related to model structures, interpretation of driving context, and assessment of risk and associated sampling issues. The methodological paradigms are founded on a formal definition of the attributes of a valid crash surrogate that can be used in model formulation and testing. After a brief summary of the type of data collected in the studies, an overall framework for the analysis and a range of specific models to test hypotheses of interest are presented. A summary is given of how the systematic analyses with statistical models can extend safety knowledge beyond an assessment of “causes” of individual crashes.

Safety Implications of Multiday Driving Schedules for Truck Drivers: A Comparison of Field Experiments and Crash Data Analysis

The detailed analysis of preexisting crash and noncrash data representing an estimated 16 million vehicle miles of travel has revealed strong consistency between crash analysis using data from the 1980s and field experiments conducted in the 1990s. Time of day of driving is associated with crash risk: night and early morning driving has elevated risk in the range of 20% to 70% compared with daytime driving. Overall, 16 of 27 night and early morning driving schedules had elevated risk. Irregular schedules with primarily night and early morning driving had relative risk increases of 30% to 80%. In addition, there remains a persistent finding of increased crash risk associated with hours driving, with risk increases of 30% to more than 80% compared with the first hour of driving. These increases are less than previously reported and are of similar magnitude to the risk increases caused by multiday schedules. Finally, there is some evidence, although it is far from persuasive, that risk increases may be assoc...

Safety Effectiveness of Leading Pedestrian Intervals Evaluated by a Before-After Study with Comparison Groups

Strategies to reduce pedestrian–vehicle crashes at intersections should be investigated. Implementation of the leading pedestrian interval (LPI) has been recommended as a strategy for reducing pedestrian–vehicle crashes at signalized intersections; however, research on quantification of the safety effects of the LPI has been limited. Site characteristics, traffic volumes, pedestrian volumes, and crash data were obtained for 10 signalized intersections where the LPI was implemented in State College, Pennsylvania. Similar data were obtained for 14 stop-controlled intersections within the State College area. A before–after with comparison group study design was used to evaluate the safety effectiveness of the LPI implementations. The results suggest a 58.7% reduction in pedestrian–vehicle crashes at treated intersections, which is statistically significant at the 95% confidence level. An economic analysis was also conducted to determine the cost-effectiveness of the strategy. Given the low cost of this strategy, only a modest reduction in crashes is needed to justify its use economically. On the basis of the estimated safety effectiveness, the necessary crash reduction is easily achievable.

Crash modification factors for changes to left-turn phasing

This study estimated crash modification factors (CMFs) from before–after evaluations of two treatments targeted at reducing left-turn crashes at signalized intersections: (a) changes from permissive to protected–permissive phasing and (b) the implementation of a flashing yellow arrow for permissive left turns. Results of the first evaluation—based on 59 intersections in Toronto, Ontario, Canada, and 12 in North Carolina—indicated a substantial reduction in left-turn opposing through crashes, especially at intersections at which more than one leg was treated, and a small percentage increase in rear-end crashes. For the second evaluation (the implementation of the flashing yellow arrow)—based on data from 51 signalized intersections in Oregon, Washington State, and North Carolina—the results indicated a safety benefit at locations with some kind of permissive left-turn operation before and a disbenefit at locations that had a protected-only operation before. The study estimated the standard deviation of the distribution of the CMF in addition to the conventionally estimated standard error of the mean CMF value. For several CMFs, the standard deviation of the distribution was larger than the standard error of the mean value of the CMF and indicated a substantial variation in the effect of the treatment across different sites. This finding indicates the need for further research into the development of crash modification functions instead of CMFs and for the use of large treatment databases to undertake a more extensive disaggregate analysis of the safety effects. The finding also emphasizes the importance of providing a more explicit consideration of CMF variability in future editions of the Highway Safety Manual.

Safety evaluation of offset improvements for left-turn lanes

This study is a safety evaluation of offset improvements for left-turn lanes, a treatment intended to reduce the frequency of crashes by providing better visibility for drivers who are turning left. Geometric, traffic, and crash data were obtained for installations in Nebraska, Florida, and Wisconsin and for a number of untreated reference sites in each state. To account for potential selection bias and regression to the mean, an empirical Bayes before–after analysis was conducted. There was a large difference in observed effects in the three states, which may be explained, in part, by the variety of offset improvements applied. Florida and Nebraska employed pavement-marking adjustments or minor construction to improve the offset, but most improvements did not result in a positive offset. Wisconsin, conversely, reconfigured left-turn lanes through major construction projects and realized significant positive offsets. Wisconsin showed significant reductions in all crash types investigated (total reduction, 34%; injury, 36%; left turn, 38%; and rear end, 32%), while results in Florida and Nebraska showed little or no effect on total crashes. For Nebraska, however, a disaggregate analysis did reveal that the percentage reduction in crashes increases as the expected number of crashes increases. An economic analysis indicated that offset improvement through reconstruction is cost-effective at intersections with at least nine expected crashes per year and in which left-turn lanes are justified by traffic volume warrants.

Evaluation of Safety Strategies at Signalized Intersections

This report presents crash modification factors (CMFs) for safety strategies at signalized intersections. CMFs are a tool for quickly estimating the impact of safety improvements. The report will be of particular interest to safety practitioners responsible for programming and implementing highway safety improvements at intersections.

Two Low-Cost Safety Concepts for Two-Way, Stop-Controlled Intersections in Rural Areas

FHWAs Office of Safety has identified intersections as one of its safety focus areas. As part of FHWAs efforts to reduce intersection crashes and related injuries and fatalities, two low-cost concepts have been identified: (a) lane narrowing on major road intersection approaches and (b) installing channelizing islands and supplemental stop signs on minor road intersection approaches. With the cooperation of several transportation agencies, these strategies were deployed at a limited number of sites in the United States; specifically, the concepts were deployed at two-lane, two-way, stop-controlled intersections. Positive operational and safety effects were observed with the installation of the two strategies. The lane-narrowing concept was shown to significantly reduce speeds and crashes on the major road approaches. The average speed reductions for all vehicles (3.5 mph) and for trucks only (4.4 mph) were statistically significant at the 95% confidence level. Total, fatal and injury-only, and angle crashes were reduced after deployment, but only the reduction in fatal and injury-only crashes was significant at the 95% confidence level. The minor road channelizing concept is shown to improve driver compliance with traffic control and to reduce speeds and crashes on minor approaches. The average speed reductions on minor approaches (nearly 10 mph) were statistically significant at the 95% confidence level. The crash rate for total, fatal and injury-only, angle, and rear-end crashes decreased after deployment.