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In general, the use of superelevation and changes in a road's transverse slope are typically based on the road's design speed to provide safety and comfort for a vehicle driving on a horizontal curve. However, due to the difference between operating and design speeds, there has always been uncertainty in determining the margin of safety using superelevation. This paper discusses the assessment of the safety margins obtained from the application of superelevation in horizontal-curve design. The assessment uses geometric design guides and a reliability index to determine any uncertainties in the design parameters. In addition, adjusted design charts of superelevation values related to the radius of horizontal curves at various levels of probability of noncompliance are provided. The results show that the adjusted superelevation values are generally greater than those derived from current standards. The adjusted design charts can help designers select an appropriate superelevation value for a particular horizontal curve for highways that have geometric constraints. These adjusted charts may also aid designers in predicting the consequences and safety margins associated with the selection of superelevation alternatives required to approve geometric designs that involve violations of the standard requirements due to environmental constraints. [ABSTRACT FROM AUTHOR]

This paper presents an extensive investigation regarding the impacts of green signal countdown devices (GSCD) on the intersection safety and efficiency, based on field observation of critical driver and vehicle related parameters at two similar intersections (one with GSCD and the other without GSCD) in Shanghai. Statistical analysis results have revealed that the installation of GSCD can (1) encourage drivers to pass the stop-line during the amber time with higher speeds and thus result in better utilization of the amber time and increased capacity of the intersection approach; (2) smooth the driver's response to the phase transition and effectively prevent the sudden change of speeds; (3) effectively eliminate the intersection dilemma zones by allowing the drivers to envision the phase transition and make decisions in advance; and (4) significantly reduce the number of red-light violations. Despite its effectiveness in facilitating the driver decision-making process, the installation of GSCD may adversely increase the possibility of collisions with unexpected crossing vehicles or pedestrians due to the significantly increased speeds of vehicles approaching the amber time. The analysis results from this study will offer the basis for traffic practitioners, researchers, and authorities in China to assess the impact of GSCD with respect to its operational and safety performance, and propose guidelines for its proper field installation. [ABSTRACT FROM AUTHOR]

Traffic-responsive plan selection (TRPS) control is one of the largely underutilized control features in National Electircal Manufacturers Association traffic controllers. This control feature allows the controller to change timing plans in arterials or networks in response to changes in traffic patterns automatically, and can therefore result in better performance of the signal system. However, there is limited number of studies that document the procedure of TRPS configuration. The authors have previously published general guidelines on TRPS system setup and configuration, laying out a comprehensive approach that assumes general traffic patterns, a standardized arterial network, and some generic system detector locations. This paper fills a significant gap in literature by providing an illustrative procedure of actual field implementation of a TRPS system in Mexia, Texas. The paper demonstrates the configuration methodology on an arterial network that does not conform to the standardized guidelines. This implementation resulted in up to 75% reduction in delay and up to 33% increase in the lower arterial speed. In this paper, the authors provide some findings and insights into the TRPS configuration process, highlighting some solutions for potential field problems that the engineer might face when configuring a TRPS control system. [ABSTRACT FROM AUTHOR]

The addition of interchanges, traffic signals, or unsignalized driveways to an arterial corridor increases crash risk and decreases travel speeds. As adjacent vacant land is developed, however, private landowners often desire direct access to the arterial route. Because the Virginia Department of Transportation has the authority to allow direct access provided no immediate adverse impact would occur, arterials in high-growth areas tend to have an increasing number of intersections as demands for direct access grow. This study investigated the feasibility of using the provisions of the National Environmental Policy Act (NEPA) to limit corridor access (e.g., grade-separated interchanges or at-grade intersections) through four administrative mechanisms: (1) including the limited access requirement as a condition in the record of decision; (2) effecting a contract (the Federal-Aid Project Agreement) that stipulates limiting access as a requirement; (3) implementing a supplemental agreement between interested parties that indicates how the character of the corridor shall be maintained; and (4) documenting the corridor-preservation conditions that remove the requirement of an Environmental Impact Statement. Factors that limit the effectiveness of these mechanisms include (1) the source of funds for the project; (2) the extent to which environmental concerns remain constant after the NEPA process is concluded; (3) the willingness of agencies to exercise the authority they are accorded through NEPA-related agreements; and (4) the passage of congressional legislation that enhances or restricts the scope of NEPA. Case studies of projects in Virginia where the NEPA process has helped preserve the characteristics of corridors show that the process may serve to raise barriers to adding access points but does not guarantee that access points may never be added. [ABSTRACT FROM AUTHOR]