Modeling Signalized-Intersection Safety with Corner Clearance Xuecai Xu, Ph.D. 1 ; Hualiang (Harry) Teng, Ph.D. 2 ; Valerian Kwigizile, Ph.D. 3 ; and Eneliko Mulokozi 4 Abstract: Signalized intersections next to each other on the same arterial share some unobservable information, such as traffic flow and roadway characteristics. This study investigated the impact of access management techniques on crash counts at signalized intersections. The analysis was performed using crash data from 275 signalized intersections in southern Nevada. The panel data random-effect model was used to account for the unobserved factors for each unique arterial. It was found that the negative binomial (NB) regression models were the best in reflecting the dispersion in the crash data. Therefore, the random-effects negative binomial model (RENB) was applied to investigate the relationship between crash occurrence and access-management techniques. The results of the panel data RENB models were compared with those from the pooled NB models, which did not account for the panel data structure. Evaluation of the goodness-of-fit of the models developed indicated that the random-effect negative binomial model was the best-fit for the data at hand. The results from the panel data RENB showed that nine variables significantly affecting the safety at signalized intersections were the average length of corner clearance, traffic flow, land-use types, number of left-turn lanes for main streets, number of through lanes for main and minor streets, posted speed limit on main and minor streets, and grades of legs. DOI: 10.1061/(ASCE)TE.1943-5436.0000636. © 2014 American Society of Civil Engineers. Author keywords: Access management; Signalized intersection; Count data model; Panel data model. Introduction Signalized intersections are areas where traffic goes through or changes route by responding to a traffic control (if any), such as traffic signals. Too much access around intersections can cause traffic conflicts and lead to crashes. In efforts to enhance safety around signalized intersections, access management techniques have been implemented to control access and improve safety (Xu 2010). Corner clearance is one important measure to control proximity of the access from intersections. It is defined as the minimum length between a signalized intersection and the first driveway along connecting streets. It is important to keep the corner clearance long enough to separate conflict points effectively. By providing sufficient corner clearance, it gives drivers enough time to make safe maneuvers. When corners are not adequately cleared, the conflicts between turning and through traffic are imminent, which may lead to crashes and longer delays. A literature review revealed that only a few studies have explored the impact of corner clearance on intersection safety and operation. McCoy and Heimann (1994) evaluated the impact of corner clearance on saturation flow rates at signalized intersections in Lincoln, Nebraska. Long and Cheng-Tin (1993) and Kaub (1994) estimated the required corner clearance distances by developing a formula, while Gluck et al. (1999) focused on how to specify the corner clearance criterion for practical implementa- tion. A study by Oh et al. (2004) was the only one in which the corner clearance impact on safety at intersections was studied. Although driveways at intersections were considered in their study, only the number of driveways at intersections was included, not the corner clearance. In concept, an intersection approach may have more than one driveway. However, the corner clearance could vary significantly given a fixed number of driveways on approach. Thus, it is necessary to consider corner clearance directly in the evaluation of intersection safety in addition to other factors, such as traffic flow and roadway function classifications that have been included in the study by Oh et al. (2004). It was also found that the study by Oh et al. (2004) was for intersections in rural areas. The corner clearance requirements in urban areas would be different from the rural areas. The objective of this study is to identify the influence of corner clearance and other factors on safety at intersections. In the inves- tigation, this study considers the spatial correlation of intersection along the same arterial. Such intersections are most likely sharing some unobservable factors that are unique to an arterial. For exam- ple, through traffic flow along one arterial has similar patterns, and adjacent intersections probably share similar land-use types and roadway design features. This is especially common in urban areas. Such unobserved similarities may lead to a statistical problem known as heterogeneity, which may lead to invalid statistical infer- ence if ignored. To address this issue, a panel data model is adopted in this study where the intersections on an arterial are vie.wed as a group. A unique intersection is imagined existing for an arterial, and the observations for the intersections on the same arterial are viewed as repeated ones for the virtual unique intersection. To evaluate the safety impact of corner clearance at intersections, 1 Lecturer, School of Civil Engineering and Mechanics, Huazhong Univ. of Science and Technology, Wuhan 430074, China. E-mail: xuecai _xu@hust.edu.cn 2 Associate Professor, Dept. of Civil and Environmental Engineering, Univ. of Nevada, Las Vegas, NV 89154. 3 Assistant Professor, College of Engineering and Applied Sciences, Western Michigan Univ., Kalamazoo, MI 49008 (corresponding author). E-mail: Valerian.Kwigizile@wmich.edu 4 Graduate Student, Dept. of Civil and Environmental Engineering, Univ. of Nevada, Las Vegas, NV 89154. Note. This manuscript was submitted on January 10, 2012; approved on October 14, 2013; published online on March 12, 2014. Discussion period open until August 12, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Transportation En- gineering, © ASCE, ISSN 0733-947X/04014016(11)/$25.00. © ASCE 04014016-1 J. Transp. Eng. J. Transp. Eng. 2014.140. Downloaded from ascelibrary.org by Huazhong University of Science & Technology on 05/21/14. Copyright ASCE. For personal use only; all rights reserved.