Enhancing Safety and Operations at Complex Interchanges With Improved Signing, Markings, and Integrated Geometry

Executive Summary

The modern highway system has experienced considerable changes since the interstate system and other limited access highways were first designed and constructed in the mid-1900s. Population movement outside of cities and the increased movement of passenger and freight traffic have led to increased congestion and necessitated new approaches for maintaining mobility.

Mobility and capacity enhancements, including additional lanes, preferential lanes and ramps, and multilane exits have subsequently complicated the design and operation of interchanges. Drivers approaching an interchange must undertake the navigation task while workloads from the guidance and control tasks are particularly high. In addition to the task of selecting a lane appropriate to the desired route, which typically requires lane changes, drivers experience workload demands related to conflicting traffic from lane changes associated with entering and exiting maneuvers. The combined workload of guidance and control tasks related to collision avoidance and navigation is easily exacerbated by a reduction in available time to make a maneuver (due to speed or short distances between critical points) and factors associated with roadway geometric design, roadway cross section, and traffic volumes and density.

The purpose of this study was to develop recommendations for signing, delineation, and geometric design that will reduce workloads at critical points approaching interchanges that exhibit a high degree of complexity. In the development of these recommendations, the following activities were completed:

• Identification of attributes influencing interchange complexity.
• Evaluation of current geometric design, signing, and marking practices.
• Simulator study investigating driver behavior at different interchange layouts.
• Field study investigating real-world driver behavior at complex interchanges.

The project team conducted a literature review to understand previous research on interchange complexity and identify elements that influence the complexity of an interchange. Previous research and feedback from stakeholders identified specific scenarios that make an interchange (and common challenges associated with them) complex as well as practices that have been implemented to try to address these challenges. As an outcome of the literature review, a list of nearly 200 characteristics that contribute to complexity was defined, and this list was distilled into 10 topics for further review.

Building on the literature review, the project team conducted a practices evaluation to determine, by means of site visits and a scan of photographs and videos available to the project team, the variations in the application of engineering design undertaken by various States. Differences related to geometric design, traffic control devices (TCDs), pavement markers, delineation, and raised reflective pavement markers were captured and discussed.

The project team conducted a simulator study to experimentally evaluate driver lane selection in complex interchange situations. Complex interchanges typical of the existing field applications were designed, and multiple alternative approaches to guide signing were developed for four interchange layouts. The effectiveness of driver decisionmaking was evaluated in terms of both whether drivers make accurate lane choices and the potential impacts to safety and efficiency.

The simulator study included a sample of 121 research participants (60 male and 61 female) in 3 geographic areas: Orlando, Florida; Myrtle Beach, South Carolina; and Gainesville, Virginia. Participants were found to be accurate regardless of the signing approach used. Similarly, participants seemed to understand the signing alternative—that is, in general, there was an average of less than one unnecessary lane change (ULC) per interchange. Together, the high accuracy presented by participants and few ULCs indicate that drivers tend to understand a series of guide signs leading up to complex interchanges as long as the interchanges are designed consistently and with good signing practices. This simulator study also found different signing approaches affected where participants tended to make their lane changes; this information can be useful when designing interchanges as it could have implications on safety and operational issues.

To complete the field study, the project team collected data from six complex interchanges across the United States. Three types of data were collected for this field study: photographs, videos from fixed-location cameras, and videos from unmanned aerial vehicles. Findings from the field study highlight common behavior as drivers approach complex interchanges. No major safety issues were observed through the field study. One common finding across all sites was that exiting traffic was found to most commonly use the exit-only lane rather than the option lane. In addition, few common behaviors identified through the field video show last-minute lane changes; drivers typically entered their target lane well upstream of the interchange.

The project team’s efforts in the various activities led to six key findings:

• Consistent application of signing principles, both among locations and within various geometric design scenarios, leads to correct driver responses.
• The existence of explicit technical policy typically results in improved consistency in signing, pavement markings, and geometric design.
• A well-developed pavement marking and delineation policy generally results in appropriate application of pavement marking patterns.
• The consistent use of arrows on guide signs appears to correspond with a design that correlates with intention in the signing of freeway-grade facilities and is generally indicative of fewer design and fabrication errors in the field.
• Providing specific guide signing with corresponding appropriate delineation appears to reduce the likelihood of roadway departures and abrupt lane changes.
• A uniform application of warning signs for lane reductions, for both mainline lanes and entering lanes, is lacking in many jurisdictions.

Six categories of recommendations (“treatments”; see list that follows) were identified and discussed. Each treatment is the result of understanding the interrelationships of various attributes within each research topic and the application of those relationships to practice outcomes, including those evaluated in the field study and simulator study:

• Ramp terminal arrangements.
• Sign layout: sign legend arrangement and panel configuration.
• Sign placement: arrows, distances, and relationship to geometric design.
• Delineation for exiting lanes and special use lanes.
• Lane-reduction methods, signing, and delineation.
• TCD education and design review workshops.

In this report, the project team describes each treatment with examples of undesirable practices and anticipated and observed outcomes, provides existing design guidelines with a general perspective on implementations in multiple jurisdictions, outlines the primary principles of the concept, provides application examples, provides specific recommendations to address undesirable practices, and summarizes the breadth and depth of implementation options.