Improve Sight Distance at Signalized Ramp
Terminals of Partial-Cloverleaf Interchanges to Deter Wrong-Way Entries
Jin Wang, S.M.ASCE1; Huaguo Zhou2; and Yunan Zhang3
Abstract: Providing drivers with open sight distance on entrance ramps can help prevent wrong-way entries at interchange terminals. For partial-cloverleaf (parclo) interchange terminals with close-spaced two-way ramps, appropriate intersection balance and median barrier lengths can provide motorists with a better view of entrance ramps and a better turning radius. A current best intersection balance practice by Washington State Department of Transportation (WSDOT) is to locate the stop line at no more than 60% of the way through the intersection. However, this has not been proven for safety benefits. To fill this void, this paper provides a comprehensive evaluation of the impact of stop line positioning based on wrong-way crash data analysis. Effects of different lengths of the median barriers used to separate two-way ramps on left-turn driver sight distances were also investigated by analyzing driver perspectives of roadway information with a threedimensional (3D) simulation and analytical methodology. Conclusively, the current best practice was proven effective in deterring wrong-way driving, and general guidelines for appropriate median barrier lengths on two-way ramps were developed. DOI: 10.1061/JTEPBS.0000039.
© 2017 American Society of Civil Engineers.
Introduction
Wrong-way driving (WWD) is a major traffic safety hazard at interchange terminals, especially when exit ramps and entrance ramps are closely spaced. Providing drivers with open sight distance on entrance ramps can significantly reduce wrong-way movements. An adequate sight distance not only provides drivers on crossroads with a better view of ramp terminals but also helps drivers distinguish between entrance and exit ramps at night with headlights. Thus, appropriate intersection balance throughout the entire width of the intersection has decisive effects on the sight distance at intersections. At the intersection of two-way ramps and crossroads for a partial-cloverleaf (parclo) interchange, a desirable throat width can be achieved by moving stop lines for left turns from the crossroad forward so that motorists have a better view of the entrance ramp and a better turning radius. The Washington State Department of Transportation (WSDOT 2013) proposed that the distance between the stop line for left turns from a crossroad and the middle of the median separating on and off ramps be no more than 60% of the entire intersection width (Fig. 1). In this study, intersection balance is defined as the percentage through the intersection that the left-turning point is found.
Guardrails or concrete median barriers are often used to separate adjacent entrance and exit ramps at parclo interchanges. Past studies (Morena and Leix 2012) have shown that guardrails may block the view of entrance ramps for left-turn drivers when they extend to
1 Graduate Research Assistant, Dept. of Civil Engineering, Auburn Univ., Auburn, AL 36849-5337 (corresponding author). E-mail: jinwang@ auburn.edu 2 Associate Professor, Dept. of Civil Engineering, Auburn Univ., Auburn, AL 36849-5337. E-mail: zhouhugo@auburn.edu 3 Associate Professor, School of Architecture and Design, Beijing Jiaotong Univ., Beijing 100044, China. E-mail: ynzhang1@bjtu.edu.cn Note. This manuscript was submitted on February 1, 2016; approved on November 1, 2016; published online on February 22, 2017. Discussion period open until July 22, 2017; separate discussions must be submitted for individual papers. This paper is part of the Journal of Transportation Engineering, Part A: Systems, © ASCE, ISSN 2473-2907. |
the stop line of the exit ramp. Drivers making a left turn onto a freeway need to see the delineation of entrance ramps, such as pavement markings, curbs, and other elements, to detect the correct path. Although median barriers used on two-way ramps can prevent catastrophic median crossover collisions and help direct vehicles onto the correct roadway, they can also create sight obstacles and increase the likelihood of WWD if they are overly long. Fig. 2 shows an example of a median barrier (guardrail) blocking the driver’s view of the throat of an entrance ramp in Michigan. Thus, it is important to properly design the median barrier far enough from the terminal so that a left-turning driver can recognize the entrance ramp.
The objective of this study was to evaluate the safety effects of the existing guideline on interchange balance by WSDOT and to develop new general guidelines for median barrier design to improve the driver’s view of entrance ramps at signalized ramp terminals of parclo interchanges.
Literature Review
This section reviews existing practices and studies on intersection layout, especially left-turn stop line positioning, three-dimensional (3D) views by left-turn drivers, and the impact of median barrier design on the driver’s view of entrance ramps.
Stop Line Positioning
The Manual on Uniform Traffic Control Devices (MUTCD) (FHWA 2009) indicates that “if used, stop lines shall consist of solid white lines extending across approach lanes to indicate the point at which the stop is intended or required to be made. Stop lines should be used to indicate the point behind which vehicles are required to stop, in compliance with a STOP (R1-1) sign, traffic control signal, or some other traffic control device. If used, stop and yield lines should be placed a minimum of 4 feet in advance of the nearest crosswalk line at controlled intersections, except for yield lines at roundabouts as provided for in Section 3C.04 and
Fig. 1. Current best practice of the parclo int
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