Safety Evaluation of Corner Clearance at Signalized Intersections

CHAPTER 1. INTRODUCTION

BACKGROUND ON STRATEGY

Corner clearance is defined as the distance between an intersection and the nearest driveway or access point along the approach. Adequate corner clearance is an important factor in the safety and operations at intersections. AASHTO’s A Policy on Geometric Design of Highways and Streets (also known as “The Green Book”) notes that driveways should not be located within the functional area of an at-grade intersection or in the influence area of an adjacent driveway.⁽¹⁾ However, the presence of conflicting driveways within the functional area is often unavoidable, especially in urban environments. Limited corner clearance, or the presence of driveways in proximity to intersections, is suspected to have negative effects on operational efficiency, capacity, and safety due to driveway turning movements conflicting with vehicles at the larger intersection.

While inadequate corner clearance is a concern for all types of intersections, signalized intersections develop recurring queues within the functional area of the intersection that can lead to conflicts with vehicles turning into and out of driveways. Approaches to signalized intersections also have more lanes on average than other types of at-grade intersections, which can cause difficulties for drivers leaving driveways to weave and maneuver into their desired lanes.

Figure 1 shows a photo of a signalized intersection with limited corner clearance. Refer to the appendix for further examples of intersections included in this study.

©VHB.

Figure 1. Photo. Signalized intersection with limited receiving corner clearance.

States have proposed access management strategies to balance the safety and operational efficiency of intersections while maintaining access to properties along and adjacent to the roadway. National Cooperative Highway Research Program (NCHRP) Report 500 Guide, Volume 12: A Guide for Reducing Collisions at Signalized Intersections, notes that improving access management near signalized intersections is one of seven objectives for improving the safety of signalized intersections.⁽²⁾ Inadequate corner clearance is often a reason why access management strategies are proposed at intersections during safety reviews. However, there is limited information available about the quantitative safety effects of corner clearances. This study serves to address the need for research into the safety effects of corner clearances on the mainline approach and receiving corners at four-leg, signalized intersections. Figure 2 shows a general layout of a study site in this evaluation, illustrating the measurement of corner clearance and defining mainline approach and receiving corners.

Source: FHWA.

Figure 2. Schematic. General layout of study site.

BACKGROUND ON STUDY

The Federal Highway Administration (FHWA) established the Development of Crash Modification Factors (DCMF) program in 2012 to address highway safety research needs for evaluating new and innovative safety improvement strategies by developing reliable quantitative estimates of their effectiveness in reducing crashes. The ultimate goal of the DCMF program is to save lives by identifying new safety strategies that effectively reduce crashes and to promote those strategies for nationwide implementation by providing measures of their safety effectiveness and benefit–cost (B/C) ratios through research. State transportation departments and other transportation agencies need objective measures for safety effectiveness and B/C ratios before investing in broad applications of new strategies for safety improvements. Forty State transportation departments provide technical feedback on safety improvements to the DCMF program and implement new safety improvements to facilitate evaluations. These States are members of the Evaluation of Low-Cost Safety Improvements Pooled Fund Study, which functions under the DCMF program.

LITERATURE REVIEW

The research team conducted a literature review concentrated on the safety effects of corner clearance distance as well as access spacing and various management strategies regarding property access. Most evaluations to date have focused on corridor safety effects rather than intersection safety. The following provides a summary of the salient research related to specific strategies.

Kwigizile et al. examined changes in the number of crashes at urban signalized intersections as a result of corner clearance and other variables.⁽³⁾ A zero-inflated negative binomial (ZINB) model was selected from four model forms as the best model for determining the safety effects of the treatment. The authors modeled corner clearance as the number of corner clearances (i.e., number of access points) and the average corner clearance in feet, with a maximum of 250 ft. Table 1 shows the ZINB model results.

The results indicate that increased corner clearance and fewer access points yield fewer crashes. Commercial driveways with limited corner clearance led to higher crash rates than residential access. Signals with higher minor road volumes had a higher number of crashes. Crashes generally increased with the addition of left-turn lanes and through lanes, with through lanes leading to a greater increase.

Table 1. ZINB model estimation results adapted from Kwigizile et al.⁽³⁾