U.S. Department of Transportation
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
|Publication Number: FHWA-HRT-13-048 Date: October 2013|
Publication Number: FHWA-HRT-13-048
Date: October 2013
This project yielded several overall conclusions related to driver expectations at complex interchanges. These conclusions are organized around the following two primary objectives of this project:
The findings related to each objective are discussed in the following sections.
The activities conducted in this project provided information about several aspects of this question. These aspects are discussed in the following sections.
Definition of Driver Expectations
Based on the literature review, it was clear that the concept of driver expectations has not been thoroughly defined with specific regard to interchanges and common driver maneuvers at interchanges. However, this concept has received consideration with regard to broader driving tasks. There are several existing definitions of driver expectations.(12,3,13) The majority of definitions that were found typically included some variation of the comprehensive definition provided by Lunenfeld and Alexander , which states that expectancy is "a driver's readiness to respond to situations, events, and information in predictable and successful ways."(pg. 153)(3) Some of the attributes of driver expectancy include the following:
The empirical findings regarding driver expectations are as follows:
Key Driver Expectations Regarding the Navigation of Complex Interchanges
The primary conclusions related to driver expectations come from the focus group discussions described in chapter 3. Some of the conclusions are specific to individual scenarios covered in the focus groups, while others are relevant to more than one scenario. The conclusions include the following:
How Various Interchange Elements Affect Driver Tasks and Performance
This question was primarily addressed in the task analysis described in an earlier project report.(4) This analysis provided high-level information about how some interchange elements have the potential to affect driving, and the relevant findings are briefly summarized for various interchange elements as follows:
Key Challenges to Drivers
The task analysis also indicated that drivers face several key challenges in interchange driving. These challenges include the following:(4)
Safety and Capacity Implications of Driver Expectations Findings
The findings from this project illustrate some of the implications of driver expectations on safety and capacity at complex interchanges. Effects related to safety and capacity are discussed in the following subsections. It should be noted that this information is primarily based on qualitative focus group discussions or empirical data from a limited set of drivers. Therefore, these findings should be viewed as suggestive of the driver expectations that could impact safety or capacity, but their link to actual traffic data is not confirmed in any way in this project.
The concerns related to safety generally involve drivers at complex interchanges having to execute multiple actions with limited time, which can lead to drivers making more mistakes in situations that generally have lower margins for error. Some of the specific safety-related implications are as follows:
Most of the implications regarding interchange capacity involve uneven lane usage; however, effects related to drivers slowing or missing exits are as follows:
It is worth noting that the described lane usage patterns pertain mostly to drivers who are unfamiliar with the interchange. There were focus group comments suggesting that familiar drivers actually improve capacity by avoiding congested lanes when they know that other lanes will also take them to their destination. An extension of this notion is that if drivers clearly understand which lane goes where, they may make better use of less crowded lane alternatives. Therefore, if it is possible to communicate lane information sufficiently using guide signs or other approaches, even unfamiliar drivers could act as familiar drivers and optimize capacity.
The following sections summarize the key conclusions and findings regarding recommendations for sign design that were identified in this project.
Key Design Principles and Guidance from Existing Research
One outcome of the task 2 literature review was an initial set of key design principles, which include the following:
The "Results" section in chapter 2 provides additional details about specific steps that can be taken to address each design principle.
Findings Related to Sign Design from the Task 7 Data Collection Activities
The task 7 empirical activities yielded several findings that support the development of specific recommendations for guide sign design. The spatial organization and layout of guide sign information have an important influence on driver interpretation of signs and their expectations of upcoming interchange geometry. The clearest and most consistent finding from task 7 is that perceptual factors related to the organization of information on a sign influence how drivers interpret the sign. The focus group discussions also provided similar findings. These perceptual factors can involve the layout of informational elements, how sign elements are grouped, and/or the position of the sign on the sign bridge. The data suggest that some drivers formed strong expectations based on the arrangement of sign information in certain situations. This is especially apparent in topic 3 of task 7 in which sign placement on the bridge was a stronger cue than the specific sign notation for communicating a left exit. Also, there is evidence from this same topic that drivers confused left exit panels with exit only panels when those panels were located on the right side of the sign bridge. This suggests that the expectations that drivers had related to sign position biased their understanding of the sign. Under time-constrained driving conditions, these expectations can influence their reading of signs and make the difference between correctly or incorrectly interpreting a guide sign. Other specific findings related to the influence of perceptual factors are discussed in the "Conclusions" section of chapter 4.
In the focus groups, most drivers appeared to interpret down lane arrows rather narrowly to correspond to the destination that is directly above or grouped with the arrow. For example, in scenario 1, while both NW Ind. Area and St. Helens destinations could be reached by either of the two right lanes, most drivers paired each destination with only the lane directly below the corresponding lane arrow. This tendency led to problems later in the scenario because drivers associated the rightmost lane, which was an exit only lane, with St. Helens when the exit was for a different destination (Vaughn St.).
Understanding the perceptual factors that influence guide sign interpretation is important because they represent attributes that can be exploited to make signs more useful to drivers. However, if signs are designed without proper consideration of these factors, it could also lead to unnecessarily complicated or confusing guide signs. This is especially important for complex interchanges because signs typically communicate more information, the interchanges are more likely to be unique and unfamiliar to drivers, and the interchanges are uncommon and are more likely to involve atypical geometric elements.
Up arrows yield consistently better results than down arrows in terms of accurate driver understanding of permissible interchange movements and efficient option lane usage. In topics 1, 2, and 4, up arrows led to more efficient option lane usage or better comprehension of permissible movements when compared to down arrows. Up arrows differ from down arrows in multiple ways. They often have a clearer visual alignment with a single roadway lane. The movement information can be inherent in the arrow (e.g., curved to the right for a right exit or straight for the through destination). Additionally, since the curvature of the arrow can be varied by destination direction, up arrows can be grouped with other up arrows pointing the same direction (e.g., to indicate that both an option lane and an exit only lane lead to the same destination). Overall, these findings suggest that drivers require additional or specific information about option lanes and the movements they represent and that up arrows can be used more effectively to provide this option lane information.
An aspect of sign comprehension that was examined in this study was how drivers visually group destination information with other sign elements and particular roadway lanes. A key sign element used for communicating which information goes together is the type of separator used to divide destination labels on the sign. Three separators were investigated in this study: vertical lines, hyphens, and multiline separations. In addition to these, data suggest that the presence of an exit only panel (more specifically, the space it occupied along the bottom of the sign) may also have acted as a visual separator. The results found for the various separators are as follows:
Clearly, separation cues influence how drivers associate destination information with specific lanes. Although these elements were not tested exhaustively or necessarily in isolation, these findings demonstrate the importance of the choice of separator, as each led to different expectations for the upcoming roadway, particularly when used in conjunction with an exit only panel. Additionally, the exit only panel itself deserves consideration as a separator since it appeared to influence how drivers grouped destination information with lanes.
Based on the work conducted in this project, there are several recommendations for follow-up activities that can help researchers understand driver expectations at complex interchanges. Follow-up activities related to the task analysis are provided in the task 5 report.(4) Follow-up activities related to task 7 data collection are described in this section.
Additional research should be conducted to obtain a more complete understanding of the influence of perceptual factors on guide sign interpretation. The current research clearly demonstrates that perceptual factors exert a strong influence on driver interpretation and responses to guide sign information. The first conclusion in the previous section discusses the significance of this finding in detail. However, while the data collected in task 7 provides an initial appreciation of the importance of perceptual factors on driver expectations and their interpretation of guide signs, the resulting data represent an initial look at some of these factors. A more comprehensive and systematic approach is required to fully understand the role of information layout and organization on driver comprehension and interpretation of guide signs.
As mentioned previously, a key benefit of understanding the perceptual factors that influence guide sign interpretation is that they represent attributes that can be exploited to make signs more useful to drivers. Also, failing to properly consider these factors could lead to unnecessarily complicated or confusing guide signs. There are other useful applications for this type of information that a more detailed investigation of this topic would provide as follows:
Overall, the importance of perceptual factors in guide sign design is a relatively unexplored topic, yet the data from the current research indicates that it is potentially a key topic for designing guide signs that can be quickly and accurately understood by drivers. Conducting additional research in this area will support activities that will improve guide sign design and facilitate driver navigation of complex interchanges.
This project made good progress in developing methods and tools for identifying driver expectations at complex interchanges and applying the findings to generate solutions for problems that drivers encounter. A logical extension of this work is to apply this process to additional complex interchanges that are known to cause problems for drivers. Using the approaches from this project in a systematic way would also be very useful for refining these methods, developing tools that engineers could use to identify specific problems at complex interchanges, and then finding solutions for addressing them. An example of a geometry that could be evaluated is interchanges that include collector-distributor roadways. These roadways would be valuable to investigate since there is little guidance available regarding designing for driver expectations there, and they can be complicated for drivers to navigate (e.g., a single exit may need to include signage for a complex configuration of multiple future exits or interchanges that it serves).
To extend the current methodology to examine other interchanges, the following steps would need to be taken:
A key outcome from this research would be specific recommendations for signing plans that are validated by driver testing for each interchange investigated in the project. This would include specific guide sign designs that were shown to best support driver navigation and expectations. More general recommendations would be developed for related interchanges, which would be turned into a guidance document to inform new designs or improvements at other related interchanges. Another outcome would be to refine the approaches and tools developed in this current driver expectations project to make them more directly applicable and useful for solving a broader set of signing design challenges.
In several topics examined in this study, up arrows outperformed down arrows in aspects of lane usage or communicating lane assignments. This leads to the question: which characteristics of up arrows make them more effective? Although up arrows are conceptually similar to down arrows, it appears that there may be a basic difference between them that causes drivers to perceive them differently. For example, if drivers are reading the arrows from base to point, down arrows appear to be focused on assigning destinations to lanes, while up arrows start at the driver and point to where they are going. Similarly, because up arrows can show multiple curvatures or directional combinations, they can be used to communicate additional information to drivers, often in an implicit manner (e.g., using the same curvature to visually associate movements in separate lanes that reach the same destination).
Conducting research to obtain a clear understanding of how drivers interpret different aspects of up arrows could provide additional tools for communicating interchange movements in a manner that can be immediately understood by drivers. This is especially important at complex interchanges because arrows are often used in unconventional ways to communicate novel, complex information (e.g., sign sets B and B2 in topic 7).
The new information from the current project can be used to update the Human Factors Guidelines for Road Systems.(5) The publication has an entire chapter of guidelines dedicated to interchanges; however, information about the effects of various sign elements on driver expectations in complex interchanges is clearly lacking.(5) It would be valuable for roadway designers to have more information about the consequences of using certain sign elements over others in terms of the effects on driver understanding and lane selection. An immediate contribution of the current study could be to fill some of the knowledge gaps about these issues. Some information from the current study that could be applied to the Human Factors Guidelines for Road Systems includes the following:(5)
This recommendation originates from the discussion on variability in driver interpretation of guide signs. The task 7 response booklet was used to collect demographic information about driving history and experience with interchanges; however, neither this information nor information about driver demographics, such as age, were included as factors in the analyses. A simple follow-up activity for this project would be to take a closer look at the importance of these factors as predictors of driver responses. For example, professional drivers may have more effective strategies for focusing on key elements of signs while ignoring less relevant information. If this pattern was observed in the new analysis, then it could lead to sign designs that better promote the identification of key information over secondary information. There are also important questions that can be investigated, including the following:
The results from the current investigation show that there is still a substantial degree of unaccounted variance in driver responses. Obtaining a better understanding of this variance can provide additional information that can support the design of effective interchange guide signs.
Another longer-term approach for furthering researcher understanding of driver expectations at complex interchanges is to develop work plans for investigating these questions using SHRP2 naturalistic driving data. The SHRP2 naturalistic driving study is a large, on-road data collection effort that involves instrumenting approximately 3,000 drivers' own vehicles with an advanced data collection system including internal and external viewing cameras, forward radar, Global Positioning Systems (GPSs), and other sensors. Data collection is ongoing until the end of 2013, with data becoming available afterward. An important and unique aspect of SHRP2 data is that it will record a driver's behavior over a potentially large number of repeated traversals of the same interchanges. Using these data, it should be possible to develop baseline interchange navigation profiles for specific drivers using data from repeated movements over frequently traversed simple and complex interchanges. For example, one aspect of a driver's profile could reflect the degree to which that individual prefers the option lane versus an exit only lane. It might also be possible to identify factors that lead to changes in this behavior (e.g., a driver who typically prefers an exit only lane may choose the option if traffic is heavy). It should also be possible to examine the same driver at complex interchanges that they rarely traverse to determine how behavior in a comparable, unfamiliar complex interchange differs from the baseline profile. Using this approach, it may be possible to correlate features of the complex interchanges (e.g., geometry, sign information, etc.) with changes in behavior relative to baseline performance. This activity could provide information about how features of complex interchanges impacted driving behavior, and these findings could be used to provide behavioral validation of the types of findings observed using static and driving simulator data collection approaches.
It might also be possible to examine driver behavior at unfamiliar complex interchanges at a finer level of detail. For example, in-vehicle video data could provide information about when drivers look at signs and how much time they spend reading them. Other vehicle data can provide information about changes in vehicle speed as drivers prepare for movements as well as measurements of the distances at which they try to initiate lane changes. The data can also provide indications of navigation errors in the form of GPS traces that show interchange reentry and exit at different locations. Overall, the SHRP2 naturalistic driving data provide a rich and comprehensive dataset for expanding this initial work to obtain a better understanding of driver expectations at complex interchanges.