Comprehensive Analytical Investigation Of The Safety Of Unsignalized Intersections

According to documented statistics, intersections are among the most hazardous locations on roadway systems. Many studies have extensively analyzed safety of signalized intersections, but did not put their major focus on the most frequent type of intersections, unsignalized intersections. Unsignalized intersections are those intersections with stop control, yield control and no traffic control. Unsignalized intersections can be differentiated from their signalized counterparts in that their operational functions take place without the presence of a traffic signal. In this dissertation, multiple approaches of analyzing safety at unsignalized intersections were conducted. This was investigated in this study by analyzing total crashes, the most frequent crash types at unsignalized intersections (rear-end as well as angle crashes) and crash injury severity. Additionally, an access management analysis was investigated with respect to the different median types identified in this study. Some of the developed methodological techniques in this study are considered recent, and have not been extensively applied. In this dissertation, the most extensive data collection effort for unsignalized intersections was conducted. There were 2500 unsignalized intersections collected from six counties in the state of Florida. These six counties were Orange, Seminole, Hillsborough, Brevard, Leon and Miami-Dade. These selected counties are major counties representing the central, western, eastern, northern and southern parts in Florida, respectively. Hence, a geographic representation of the state of Florida was achieved. Important intersections\u27 geometric and roadway features, minor approach traffic control, major approach traffic flow and crashes were obtained. The traditional negative binomial (NB) regression model was used for modeling total crash frequency for two years at unsignalized intersections. This was considered since the NB technique is well accepted for modeling crash count data suffering from over-dispersion. The NB models showed several important variables affecting safety at unsignalized intersections. These include the traffic volume on the major road and the existence of stop signs, and among the geometric characteristics, the configuration of the intersection, number of right and/or left turn lanes, median type on the major road, and left and right shoulder widths. Afterwards, a new approach of applying the Bayesian updating concept for better crash prediction was introduced. Different non-informative and informative prior structures using the NB and log-gamma distributions were attempted. The log-gamma distribution showed the best prediction capability. Crash injury severity at unsignalized intersections was analyzed using the ordered probit, binary probit and nested logit frameworks. The binary probit method was considered the best approach based on its goodness-of-fit statistics. The common factors found in the fitted probit models were the logarithm of AADT on the major road, and the speed limit on the major road. It was found that higher severity (and fatality) probability is always associated with a reduction in AADT, as well as an increase in speed limit. A recently developed data mining technique, the multivariate adaptive regression splines (MARS) technique, which is capable of yielding high prediction accuracy, was used to analyze rear-end as well as angle crashes. MARS yielded the best prediction performance while dealing with continuous responses. Additionally, screening the covariates using random forest before fitting MARS model was very encouraging. Finally, an access management analysis was performed with respect to six main median types associated with unsignalized intersections/access points. These six median types were open, closed, directional (allowing access from both sides), two-way left turn lane, undivided and mixed medians (e.g., directional median, but allowing access from one side only). Also, crash conflict patterns at each of these six medians were identified and applied to a dataset including median-related crashes. In this case, separating median-related and intersection-related crashes was deemed significant in the analysis. From the preliminary analysis, open medians were considered the most hazardous median type, and closed and undivided medians were the safest. The binomial logit and bivariate probit models showed significant median-related variables affecting median-related crashes, such as median width, speed limit on the major road, logarithm of AADT, logarithm of the upstream and downstream distances to the nearest signalized intersection and crash pattern. The results from the different methodological approaches introduced in this study could be applicable to diagnose safety deficiencies identified. For example, to reduce crash severity, prohibiting left turn maneuvers from minor intersection approaches is recommended. To reduce right-angle crashes, avoiding installing two-way left turn lanes at 4-legged intersections is essential. To reduce conflict points, closing median openings across from intersections is recommended. Since left-turn and angle crash patterns were the most dominant at undivided medians, it is recommended to avoid left turn maneuvers at unsignalized intersections having undivided medians at their approach. This could be enforced by installing a left-turn prohibition sign on both major and minor approaches

Restricted Crossings on Rural Highways

This study examines the potential for replacing the standard intersection design at two-way stop control (TWSC) and all- way stop control (AWSC) intersections along rural highways with a roundabout or a restricted crossing u-turn (RCUT) facility. The geometry of the RCUT design prohibits left and through movements from the side road, and provides a u-turn location downstream from the main crossing. This type of facility has been implemented for rural highways extensively in both Maryland and North Carolina, as well as in limited cases in many other states such as Minnesota and Missouri, with the potential to serve as a cost-effective solution to improve roadway safety within Nebraska. The primary takeaway from the research is that both a roundabout and an RCUT design can be relied upon to lead to significant safety improvements for unsignalized intersections on rural highways, and that the decision of which one to use should factor in the potential increase in delays to the minor approach at the RCUT design if a high demand volume is anticipated (such as Dakota City), or the consideration of whether it is permissible to interrupt the flow of the major arterial through movement with a roundabout versus leaving it free-flowing with the RCUT

Rural expressway intersection safety treatment evaluations

A rural expressway is a high-speed, multi-lane, divided highway with partial access control which may consist of both at-grade intersections and grade separated interchanges. Many State Transportation Agencies (STAs) are converting rural two-lane undivided highways into expressways for improved safety and mobility; however, collisions at two-way stop-controlled (TWSC) intersections (particularly far-side right-angle crashes) on rural expressways are reducing the safety benefits that should be achieved through conversion. When the safety performance of these intersections begins to deteriorate, the improvement path typically begins with the application of several signing, marking, or lighting improvements, followed by signalization, and ultimately grade separation. Because signals hamper the mobility expressways are meant to provide and because interchanges are not economically feasible at all problematic intersections, there is a need for more design options at TWSC rural expressway intersections. Some STAs have experimented with innovative rural expressway intersection safety treatments to avoid signalization and grade separation; however, little is known about the safety effects of these designs. Therefore, the objective of this research was to document their experience with these treatments and to conduct nayve before-after safety evaluations where possible. The ten case studies included within this thesis investigate J-turn intersections, offset T-intersections, jughandle intersections, Intersection Decision Support (IDS) technology, static roadside markers, left-turn median acceleration lanes (MALs), offset right-turn lanes, offset left-turn lanes, enhanced intersection guide signing, and dynamic advance intersection warning systems. These case studies help to begin to understand the safety improvement potential of these countermeasures and start to set the stage for the development of a richer set of design options at TWSC rural expressway intersections

Safety and Operational Assessment of Rural Free Right-Turn Ramp Intersections

Free right-turn (FRT) ramps are alternative right-turn lane designs for intersecting highways. As of 2023, 79 FRT ramps exist at 68 rural highway intersections in Nebraska. FRT ramps may be located on three-legged or four-legged intersections and may be on the minor, the major, or both minor and major approaches of the same intersection. This research compared the 68 rural FRT intersections to 24 similar non-FRT rural intersections to identify differences in crash frequency and crash rate and tested for statistical significance using a two-sample t-test. Crash data were obtained for the ten- year period of 2010-2019, with a focus on crashes reported within a quarter mile of each intersection leg. Forty different comparisons were made between the FRT and non-FRT intersections, testing varying intersection legs, AADT, and location of the FRT ramp on the major, minor, or both approaches. The results of this analysis indicated a lack of any statistically significant difference in crash frequency or crash rate among the rural FRT ramp and rural non-FRT intersections. In addition to the safety analysis, a conflict analysis was conducted to analyze the vehicle interactions between right-turning vehicles at the FRT ramp intersections and non-FRT intersections. Miovision Scout video recording equipment was used to record the traffic conflicts over 72 hours at six FRT intersections of varying AADT and the number of intersection legs. Six non-FRT intersections were paired with the FRT intersections and the conflict experienced by right-turn movement on the same approach as its FRT counterpart was observed. The conflict analysis showed that non-FRT right- turns experienced higher conflicts per 1000 entering right-turning vehicles than the FRT ramp intersections. It was concluded that the presence of FRT ramps at rural intersections does not affect the crash frequency or crash rate experienced. It was also concluded that conflict is reduced between right-turning vehicles and other traffic present at the intersection when an FRT ramp is present, especially compared to non-FRT intersections where no exclusive right-turn lane is present on the major approach. It is recommended that future research assess additional operational benefits of FRT ramps, such as delay and travel time. Advisor: Aemal Khatta

ESTIMATION OF PEDESTRIAN SAFETY AT INTERSECTIONS USING SIMULATION AND SURROGATE SAFETY MEASURES

With the number of vehicles increasing in the system every day, many statewide policies across the United States aim to increase the use of non- motorized transportation modes. This could have safety implications because the interaction between motorists and non-motorists could increase and potentially increasing pedestrian-vehicle crashes. Few models that predict the number of pedestrian crashes are not sensitive to site-specific conditions or intersection designs that may influence pedestrian crashes. Moreover, traditional statistical modeling techniques rely extensively on the sparsely available pedestrian crash database. This study focused on overcoming these limitations by developing models that quantify potential interactions between pedestrians and vehicles at various intersection designs using as surrogate safety measure the time to conflict. Several variables that capture volumes, intersection geometry, and operational performance were evaluated for developing pedestrian-vehicle conflict models for different intersection designs. Linear regression models were found to be best fit and potential conflict models were developed for signalized, unsignalized and roundabout intersections. Volume transformations were applied to signalized and unsignalized conditions to develop statistical models for unconventional intersections. The pedestrian-vehicle conflicting volumes, the number of lanes that pedestrians are exposed to vehicles, the percentage of turning vehicles, and the intersection conflict location (major or minor approach) were found to be significant predictors for estimating pedestrian safety at signalized and unsignalized intersections. For roundabouts, the pedestrian-vehicle conflicting volumes, the number of lanes that pedestrians have to cross, and the intersection conflict location (major or minor approach) were found to be significant predictors. Signalized intersection models were used for bowtie and median U-turn intersections using appropriate volume transformations. The combination of signalized intersection models for the intersection area and two-way unsignalized intersection models for the ramp area of the jughandle intersections were utilized with appropriate volume transformations. These models can be used to compare alternative intersection designs and provide designers and planners with a surrogate measure of pedestrian safety level for each intersection design examined

Estimation of safety performance functions for urban intersections using various functional forms of the negative binomial regression model and a generalized Poisson regression model

Intersections are established dangerous entities of a highway system due to the challenging and unsafe roadway environment they are characterized for drivers and other road users. In efforts to improve safety, an enormous interest has been shown in developing statistical models for intersection crash prediction and explanation. The selection of an adequate form of the statistical model is of great importance for the accurate estimation of crash frequency and the correct identification of crash contributing factors. Using a six-year crash data, road infrastructure and geometric design data, and traffic flow data of urban intersections, we applied three different functional forms of negative binomial models (i.e., NB-1, NB-2, NB-P) and a generalized Poisson (GP) model to develop safety performance functions (SPF) by crash severity for signalized and unsignalized intersections. This paper presents the relationships found between the explanatory variables and the expected crash frequency. It reports the comparison of different models for total, injury & fatal, and property damage only crashes in order to obtain ones with the maximum estimation accuracy. The comparison of models was based on the goodness of fit and the prediction performance measures. The fitted models showed that the traffic flow and several variables related to road infrastructure and geometric design significantly influence the intersection crash frequency. Further, the goodness of fit and the prediction performance measures revealed that the NB-P model outperformed other models in most crash severity levels for signalized intersections. For the unsignalized intersections, the GP model was the best performing model. When only the NB models were compared, the functional form NB-P performed better than the traditional NB-1 and, more specifically, the NB-2 models. In conclusion, our findings suggest a potential improvement in the estimation accuracy of the SPFs for urban intersections by applying the NB-P and GP models

Examining the safety performance of urban/suburban arterials and freeway segments in consideration of roadway geometry and traffic control

With the sprawling of major cities and creation of suburban areas, one problem that state agencies face is the increasing congestion in suburban arterials coupled with the safety risks posed by increasing traffic volumes at traditional intersections along arterials. In the early 1960s, a new intersection design was developed and installed in the state of Michigan, where left turns at intersections were replaced by median U-turn lanes (MUTs). This study focuses on the safety performance of corridors where median U-turns (MUTs) are present along urban and suburban boulevards. The analysis is performed in two stages; first models were developed separately for assessing the safety performance, through the examination of crash frequency and type, across individual MUTs, at intersections, and along segments on which MUTs are located. Subsequently, an aggregate-level analysis is conducted to assess the safety performance of specific intersections/MUT combinations. The second stage focused on developing models for examination of sites spanning each side of an intersection including upstream and downstream MUTs. These sites were compared to sample sites with allowed traditional left turn movements. Ultimately, the results provide guidance to agencies considering the installation of such alternative intersections. Additionally, safety risks are present during work zone projects along freeways, which are essential facilities for providing mobility. The presence of a work zone generally results in both mobility and safety impacts to road users. Minimizing the adverse impacts associated with work zones has become a priority for road agencies. This study will estimate SPFs that consider freeway geometry and traffic conditions, as well as the effects of various temporary traffic control strategies such as lane shifts, shoulder closures, and lane closures. Crash modification factors were developed for work zone duration and length. Additionally, the study results provide insight on the safety impacts associated with each of the four types of lane closures

Asphalt Art Safety Study: Historical Crash Analysis and Observational Behavior Assessment at Asphalt Art Sites

Transportation infrastructure is perhaps the most visible aspect of a city's public realm -- the sidewalks and roadways we depend on daily are often as recognizable as the buildings, destinations, and people within it. As cities transform to meet evolving needs of the future, there is an increasing opportunity for streets to not only be safe and efficient, but a unique and inspiring part of the urban experience. Among other strategies to achieve that goal, public art projects coupled with improvements to transportation infrastructure, often known as "asphalt art," offer many benefits. They can create safer, more desirable streets and public spaces. They are typically inexpensive and quickly implementable, while helping cities test long-term roadway redesigns. And they help local governments engage with residents to reshape their communities.These projects, including intersection murals, crosswalk art, and painted plazas or sidewalk extensions, have existed for years and are growing in popularity in communities across the world. Though asphalt art projects frequently include specific roadway safety improvements, the art itself is often also intended to improve safety by increasing visibility of pedestrian spaces and crosswalks, promoting a more walkable public realm, and encouraging drivers to slow down and be more alert for pedestrians and cyclists, the most vulnerable users of the road.There has been considerable public feedback, anecdotal evidence, and analyses of individual locations indicating that asphalt art can have these traffic-calming benefits and encourage safer behavior. However, despite broad support from people who use and design streets, art within the public roadway network has faced regulatory hurdles in the United States and elsewhere because of concerns about compliance with current design standards and guidance that governs roadway markings. These concerns have persisted in the absence of much rigorous evaluation or published literature on safety performance of asphalt art projects.This study was conducted to address the need for impact analysis by comparing crash rates and real-time behavior of pedestrians and motorists at an array of asphalt art sites before and after the projects were installed. There are two main components to the study: first is a Historical Crash Analysis that compares crash data prior to and after the introduction of asphalt art at 17 diverse study sites with at least two years of data. The second is an Observational Behavior Assessment that compares before and after video footage of motorist and pedestrian behavior at five U.S. locations with asphalt art projects installed in 2021 as part of the Bloomberg Philanthropies' Asphalt Art Initiative. The analysis found significantly improved safety performance across a variety of measures during periods when asphalt art was installed