Boris Claros

Safety Evaluation of Diverging Diamond Interchanges in Missouri

The diverging diamond interchange (DDI) has gained in popularity since its first implementation in the United States in 2009. The operational benefits and lower costs of retrofitting a conventional diamond with a DDI have contributed to increased use of the DDI. Research on DDIs has focused primarily on the assessment of operational benefits. Formal safety evaluations of DDIs are lacking. This study aimed to fill the knowledge gap by conducting a safety evaluation of DDIs using three types of before–after evaluation methods: naive, empirical Bayes, and comparison group. Three evaluation methods were used because each involved different trade-offs, such as data required, complexity, and regression to the mean. All three methods showed that a DDI replacing a conventional diamond decreased crash frequency for all severities. The highest crash reduction (59.3% to 63.2%) was observed for fatal and injury crashes. Crashes involving property damage only were reduced by 33.9% to 44.8%. Total crash frequency also decreased by 40.8% to 47.9%. A collision diagram analysis revealed that the DDI, as compared with a diamond, traded high-severity crashes for lower-severity crashes. Whereas 34.3% of ramp terminal fatal and injury crashes in a diamond occurred in left-turn angle crashes with oncoming traffic, the DDI eliminated this crash type. One potential concern for the DDI is the possibility of wrong-way crashes, but only 4.8% of all fatal and injury crashes occurring at the ramp terminal of a DDI were wrong-way crashes. The DDI offers significant crash reduction benefits over conventional diamond interchanges.

Safety Evaluation of Seven of the Earliest Diverging Diamond Interchanges Installed in the United States

Diverging diamond interchanges (DDIs) are increasingly popular because they provide improved traffic operations and cost savings. On the basis of theory, DDIs should be safer than conventional diamonds, but previous empirical safety studies have been limited. The objectives of this work were, therefore, to conduct a broader safety evaluation of DDIs and to recommend a crash modification factor (CMF) for the conversion of a conventional diamond to a DDI. The team analyzed seven of the earliest DDIs in the United States. Four were in Missouri and other sites were in Kentucky, New York, and Tennessee. The team collected more than 28 site years of crash and other data before intersection conversion and more than 19 site years of data after their conversion. The primary analysis was before and after with comparison sites to account for trends and potential simultaneous event biases. The results showed that crashes were reduced at most of the sites, and the team recommended a CMF of 0.67, meaning that installation of a DDI to replace a diamond should reduce all crashes by 33%. The reduction in injury crashes was even larger, with the team recommending a CMF of 0.59. Other analyses indicated that DDI installation should mean a substantial reduction of angle and turning crashes, with some reduction in rear-end crashes as well, although rear-end crashes will still be the dominant crash types after DDI installation. Clearly, DDIs offer potential safety benefits, and agencies should consider them strongly as replacements for conventional diamonds.

Empirical evaluation of J-turn intersection performance: analysis of conflict measures and crashes

Most crashes on high-speed rural expressways occur at intersections with minor roads. Through and left-turn movements on minor roads are especially susceptible. One low-cost alternative for improving safety of at-grade intersections on such expressways is the J-turn. This study evaluated the effectiveness of unsignalized J-turn intersection design in Missouri with field studies, crash analysis, and traffic conflict analysis. Field studies used detailed video-derived data at a J-turn site and a control site. Crash analysis included an empirical Bayes before–after safety evaluation of five J-turn sites. The empirical Bayes analysis found that the J-turn produced a 31.2% reduction in crash frequency for all crashes and a 63.8% reduction in crash frequency for injury and fatal crashes. Annual disabling injury crashes decreased by 91.6%, and minor injury crashes decreased by 67.9%. Annual right-angle crashes decreased from 8.6 to 0.8, a 90.2% reduction. No left-turn, right-angle crashes were reported after installation of the J-turn at any study site. The average time-to-collision conflict measure was four times higher at the J-turn site as compared with the two-way stop-controlled (TWSC) site of minor road turning vehicles; this result indicated greater safety at the J-turn site. The average wait time at the J-turn site (5 s) was half that at the control site (11 s), and the average travel time at the J-turn site was approximately 1 min more than at the TWSC site. Several study findings are transferable to J-turn installations in other states.

When driving on the left side is safe: Safety of the diverging diamond interchange ramp terminals

How safe are the ramp terminals of a diverging diamond interchange (DDI)? This paper answered this question using data from DDI sites in Missouri. First, crash prediction models for ramp terminals for different crash severities were developed. These models were then utilized in the Empirical Bayes (EB) evaluation of DDI ramp terminals. Due to inconsistencies in crash reporting for freeways in Missouri, individual crash reports were reviewed to properly identify ramp terminal crashes. A total of 13,000 crash reports were reviewed for model development and EB evaluation. The study found that the DDI ramp terminals were safer than the conventional diamond signalized terminals. The DDI ramp terminals experienced 55% fewer fatal and injury crashes, 31.4% fewer property damage only crashes, and 37.5% fewer total crashes.

Site-Specific Safety Analysis of Diverging Diamond Interchange Ramp Terminals

A site-specific analysis of the safety of diverging diamond interchange (DDI) ramp terminals was conducted for the first time. Crash modification factors (CMFs) were developed for DDI ramp terminals for different crash severity levels. The ramp terminal CMFs complement the project-level DDI CMFs developed in previous research. The study used data from 20 ramp terminals in Missouri. The safety evaluation was conducted with two before-and-after observational methods: comparison group (CG) and empirical Bayes (EB). Because of inaccurate crash locations, a systematic crash location correction process was developed. An extensive review of 8,400 individual crash reports was conducted for the calibration and safety evaluation. Both before-and-after safety evaluation methods produced consistent results. The DDI design replacing a conventional diamond decreased the numbers of ramp terminal–related crashes for all severity levels. The most significant crash reduction by method was observed for fatal and injury (FI) crashes: 73.3% (CG) and 63.4% (EB). Property damage only (PDO) crashes were reduced by 21.0% (CG) and 51.2% (EB). The total crash frequency also decreased by 42.7% (CG) and 54.0% (EB). The EB CMF values for the DDI ramp terminal were 0.366 for FI crashes, 0.488 for PDO crashes, and 0.460 for total crashes. This study serves as a case study for conducting a site-specific analysis of ramp terminals or other interchange facilities. The methodology that was used is transferable and can be used to quantify the safety effects of other innovative intersection and interchange designs.

Safety Effect of Diverging Diamond Interchanges on Adjacent Roadway Facilities

Recent evidence has shown that the diverging diamond interchange (DDI) design improves the safety of the ramp terminals and the overall safety of the interchange. What is still not known is the safety effect that a DDI has on adjacent intersections and the speed-change lanes (SCLs) at freeway entrances and exits. This study addressed this void by examining DDI installations in Missouri. The early and many DDI implementations in Missouri provided a rich data set with which to conduct this study. Twelve major signalized intersections adjacent to the DDI ramp terminals were examined. Thirty-two SCL facilities, 16 freeway entrances, and 16 exits from 11 DDI sites also were examined. A manual review was done to assign 4,073 crash reports to corresponding facilities. The empirical Bayes (EB) method was used to estimate the safety effect of the DDI on adjacent facilities. No evidence showed that the DDI design had any effect, positive or negative, on the crashes that occurred at the entrance or exit SCLs. After DDI implementation, the changes were not statistically significant for SCL crash frequency, property damage only (PDO) crashes, and total crashes. For signalized intersections next to the DDI ramp terminals, the EB analysis showed a 6.5% decrease in fatalities and injuries, which was not statistically significant. The analysis also showed a 19.5% increase in PDO and a 12% increase in total crashes, albeit statistically significant only at the 90% confidence level. In summary, no strong evidence was found that DDIs affected safety, either negatively or positively, on adjacent SCLs or intersections.

Design Guidance for J-Turns on Rural High-Speed Expressways

Owing to the J-turn’s safety effectiveness, it has become a viable alternative to replace high-crash, two-way, stop-controlled intersections on high-speed expressways. National guidance on the design of J-turns on high-speed highways is limited. What is the safety effect of spacing between the intersection and the U-turn? Under what circumstances is the provision of acceleration lanes recommended? This study answered these questions through a safety assessment on the basis of (a) an examination of crashes that occurred at 12 J-turn sites in Missouri and (b) a simulation-based assessment of the effect of various design variables and traffic flows. The crash review revealed the proportions of the five most frequently occurring crash types at J-turn sites: (a) major road sideswipe (31.6%), (b) major road rear-end (28.1%), (c) minor road rear-end (15.8%), (d) loss of control (14%), and (e) merging from U-turn (10.5%). The crash rates, accounting for exposure, decreased with the increase in the spacing to the U-turn for sideswipe and rear-end crashes; J-turns with a spacing of 1,500 ft or greater experienced the lowest crash rates. The crash rates were lower for J-turn sites with acceleration lanes for minor road traffic merging onto the major road than for sites without acceleration lanes. A calibrated simulation model analysis revealed that the presence of acceleration lanes reduced conflicts for all volumes and designs, including low volumes. The simulation analysis also reinforced the crash analysis results that safety improved with an increase in spacing between the main intersection and the U-turn.

Enhancing Safety Risk Management with Quantitative Measures

At the airfield in hub airports, many activities occur that involve a range of participants, including various-size aircraft, ground vehicles, and workers. The safety management system is FAAs approach for systematically managing aviation safety. A major component of the safety management system is safety risk management (SRM), which entails analysis, assessment, and control of safety risks, including risks on the airfield. Current SRM has few specific safety models to estimate the likelihood or frequency of risks. This paper presents an example for development and incorporation of safety models into SRM. Specifically, it discusses safety models for runway incursion that use the following variables: total and general aviation operations, length of runway by type, number of taxiway intersections, snowfall, precipitation, number of hot spots, and construction activity. Categorization and processing of data were significant because each variable used could take on multiple forms, and some types of data involved review of airfield diagrams. The data used were from 137 U.S. hub airports for 2009 through 2014. For modeling, the negative multinomial distribution was used because it proved suitable for representing overdispersed data such as runway incursion frequency. Performance of the models was assessed through the goodness-of-fit measures of log likelihood, overdispersion, and cumulative residual plots. Models were developed for five severity categories of runway incursions and three types of surface events. The safety modeling approach presented here can serve as a foundation for development of other safety models that can be integrated into SRM to enable quantitative analysis of safety risks.

Safety effectiveness and crash cost benefit of red light cameras in Missouri

ABSTRACT Objective: Red light cameras (RLCs) have generated heated discussions over issues of safety effectiveness, revenue generation, and procedural due process. This study focuses on the safety evaluation of RLCs in Missouri, including the economic valuation of safety benefits. The publication of the national Highway Safety Manual (HSM; American Association of State Highway and Transportation Officials) in 2010 produced statistical safety models for intersections and spurred the calibration of these models to local conditions. Methods: This study adds to existing knowledge by applying the latest statistical methodology presented in the HSM and more current data. Driver behavior constantly changes due in part to driving conditions and the use of technology. The safety and economic benefit evaluation was performed using the empirical Bayes method, which accounts for regression to the mean bias. For the economic benefit evaluation, the KABCO crash severity scale and crash cost estimates were used. A total of 24 4-leg urban intersections were randomly selected from a master list of RLCs in Missouri from 2006 to 2011. Additionally, 35 comparable nontreated intersections were selected for the analysis. Results and Conclusions: The implementation of RLCs reduced overall angle crashes by 11.6%, whereas rear-end crashes increased by 16.5%. The net economic crash cost benefit of the implementation of RLCs was

Airfield Incursion Modeling of United States Hub Airports

35,269 per site per year in 2001 dollars (approximately