|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-13-047
Date: August 2013
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As transportation agencies struggle with adding freeway lane capacity in times of limited resources and shrinking right-of-way, new interchange designs are being built beyond the traditional diamond and cloverleaf configurations. Designs such as the diverging diamond, flyover ramps with directional splits on the flyover, left merging entrance ramps, and others can be complex and run counter to driver expectations. The FHWA MUTCD offers limited advice for the use of diagrammatic signs for left exits, freeway-to-freeway splits, and two-lane exits with option lanes.(1) The inclusion of these diagrammatic signs has been under scrutiny by the FHWA MUTCD team and members of the Guide and Motorist Information Technical Committee of the National Committee on Uniform Traffic Control Devices. In the 2009 revision of the MUTCD, arrow-per-lane diagrammatic signs were added based on research conducted for the National Cooperative Highway Research Program (NCHRP) and as part of the traffic control device pooled fund study. (See references 1 and 3-5.)
Freeway interchanges with lane drops, double-lane exits with optional lanes, and other unusual geometries have been the subject of many studies concerning signs and markings. These geometries violate driver expectations and may result in late lane changes and erratic movements near the gore. (See references 3, 4, and 6-9.) A 1996 Texas Department of Transportation (TxDOT) project reinforced the idea that drivers have a weak understanding of optional lane interchange signing, with only 50-65 percent of drivers correctly interpreting the current (conventional) method of signing.(8)
In 1993, the Texas Transportation Institute (TTI) studied lane use arrow pavement markings along with longitudinal striping at freeway lane drops utilizing both surveys and field studies.(6) The study demonstrated that the installation of lane drop markings caused drivers to move into or out of the exiting lane farther upstream than before markings were installed. Before and after studies also revealed that the number of erratic maneuvers within the study segment decreased with the installation of lane use arrow markings.
Two different studies have been conducted to evaluate guide signing at freeway lane drops with an optional exit lane using driving simulations. First, a 2003 NCHRP project using a driving simulator showed that roughly one-third of drivers made unnecessary lane changes at these interchange locations.(3) Ultimately, the study recommended diagrammatic advanced guide signs and a conventional gore sign with pull-through arrows for the through route and an exit only plaque over the exit.(3) A 2006 TTI study determined that drivers are likely to make unnecessary lane changes when an exit only plaque is present regardless of the type of sign.(7) Drivers tended to interpret the plaque as marking the only exit lane available as opposed to a lane that is forced to exit where there may also be a second optional exit lane.(7)
TTI conducted a recent project on the use of pavement marking symbols, arrows, text, and route markers.(9) Two human factor surveys were conducted to evaluate driver comprehension and preference for different in-lane pavement marking applications. Data analysis consisted of comprehension and preference identification. The studies were conducted using a laptop computer. Both video clips and still images were used to display the interchange sign and pavement marking information as appropriate for the situation. The study made recommendations about arrows, regulatory text, route markers, and cardinal direction text.
This project was designed to identify potential improvements to current marking and signing practices for complex interchanges. Because of the initial literature review task along with discussions on other ongoing work, FHWA and the research team divided the project into two studies with the following objectives:
As decided at the kickoff meeting, TTI was to perform a driving simulator study using its desktop simulator. The research team developed the signs to be tested by a combination of a review of the existing signs and complex interchanges submitted by the State transportation departments and by the guidance presented in the 2009 MUTCD.(1) Input from focus groups and a task analysis were also to be considered. The simulator studies were to be conducted in Texas with the pilot using drivers from the College Station area. Due to the rural nature of the city, this population represented drivers unfamiliar with more complicated interchange designs typically found in urban areas. The studies were to be conducted both in College Station and in Houston to test drivers with different levels of experience with freeway driving.
The concept of complexity as it relates to an interchange and drivers’ decision process to navigate the interchange can be applied in multiple ways; therefore, it was necessary to establish a definition of complexity and identify ways to quantify its effects on the driving public. Researchers believed that developing a tool that incorporated these elements would provide a list of factors that a reviewer could score based on a review of site plans, and the presence of those factors at the interchange could be translated into a numerical complexity score. Thus, study 2 was focused on developing such a tool.
A list of potential topics was created as a guideline for the types of sign sequences needing evaluations. The initial list included the following:
This list was later altered, and topics were assigned using photographs, a TTI simulator study, or both. When deciding which topics should be addressed by the contractor survey and which by the TTI survey, researchers first considered the measures of effectiveness. For topics where it was important to determine how quickly a driver would make a lane choice, a simulator study was desired. If it was only important to know what lane drivers would choose, the contractor survey was a more appropriate and less expensive option. Also, if it was important to see signs in a sequence and for the driver to see their spatial placement on the roadway, the simulator study was more appropriate. While the simulator offered driver immersion and driving data, the laboratory study allowed for quick, inexpensive testing of many different SSs. It is important to note that finalizing the topic list was an iterative process with the division of topics between the TTI simulator study and the contractor survey, as well as the elimination and addition of particular topics.
This report is organized into the following chapters:
Topics: research, safety
Keywords: research, safety, Complex Interchanges, Signs, Simulator Study, Spreadsheet Tool
TRT Terms: research, Safety and security, Safety, Transportation safety