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

Chapter 3. Attributes Contributing to Complexity

The project team developed a comprehensive list of attributes that contribute to interchange complexity. These 210 attributes were generally related to geometric design, interchange configuration, and some driver-expectancy and driver-comfort factors. From this list, the project team identified 10 topic areas into which the attributes could be grouped, providing a framework for the refinement of potential study sites.

Development of Attribute List

The project team examined interchanges throughout the United States and Canada and identified more than 200 attributes that contribute to complexity. This examination considered prior project research, the literature, experience in design and operations, and in particular, a comprehensive examination of interchange design and operations practices with a particular emphasis on inconsistent applications.

The process for developing the treatments started with small, discrete pieces (attributes contributing to complexity) and moved into assembling those pieces into topics that could be addressed with research efforts. Based on the results of those research efforts, six individual treatments with specific applications were developed.

As an example, the hierarchal organization of attribute 4241 (exit preceding downstream exit only from same lane) is illustrated in figure 4. Each discrete attribute is assigned a four-digit code to aid in organizing the attributes and creating a useful tool for future research activities. The attribute list is divided into seven top-level groups. Within each group, subgroups, described with the nomenclature of “categories,” are included to provide for a hierarchy and organization within the groups. In this example, the group is geometric design and the category is ramp terminal arrangements. The categories are defined by the second digit of the attribute code. The third digit is used to identify the trait, which can stand alone or be described as a trait set when it is the hierarchal grouping for multiple attributes. In this example, exit ramp terminal arrangements is the trait. The final digit identifies the specific attribute, which is 4241, exit preceding downstream exit only from same lane.

Source: FHWA.

Figure 4. Graphic. Hierarchal organization of attribute 4241.

The project team noted that there are three types of attributes: characteristic attributes, contributing attributes, and mitigating attributes. Characteristic attributes are simply characteristics of an interchange, such as the presence of an option lane, and may not necessarily be indicative of a complex interchange, whether alone or even in combination with other attributes. The other two types of attributes, however, are indicative of complexity. Contributing attributes (e.g., inconsistency in control cities (2140) or closely spaced exit ramp terminals (4213)) do contribute to complexity to some degree. The interactions between these attributes may cause complexity to a degree that is greater than the sum of the individual contributions to complexity. Mitigating attributes, on the other hand, are characteristics of an interchange that generally relieve complexity, and include attributes such as use of dotted extension (5212), when applied consistently and in conjunction with related elements that support the characteristic. The improper application of a characteristic, however, can create a contributing attribute; therefore, mitigating attributes must be understood in the context of correct and consistent applications. The issue of consistency in applications is further explored in chapter 5.

Group 1000—Impacts and Outcomes

The traits in group 1000 help categorize the impacts and outcomes of TCDs as measures of traffic operations and system performance. These traits are not descriptive of the system’s built environment, but rather, its operation. The traffic operations attributes (category 1100, see table 7) describe various aspects of traffic operations, particularly those related to flow theory and performance.

Category 1200 (see table 8) continues traffic operations characteristics with an emphasis on those related to congestion. Additional characteristics related to user performance and information processing have been developed in other work, and future research in this area should examine the effects of various information sources on user reactions that affect traffic flow.

Category 1300 (see table 9) deals with the general causes and outcomes of crashes. Additional future research on complex interchange characteristics that contribute to crashes will further develop the elements of this category.

In addition to the distraction of information processing, users also experience distraction from other factors related to complexity and attendant congestion. These factors, generally categorized as “inconvenience” in category 1400 (see table 10), are socio-psychological in nature and may be difficult to measure.

The group 1000 attributes are both contributing and characteristic and generally relate to contributing and mitigating user characteristics from group 2000.

Group 2000—User Characteristics

Group 2000 generally addresses user characteristics and includes attributes that influence user perception and reaction while not explicitly addressing those attributes that are indicative of the user’s reaction to complexity.

Category 2100 (see table 11) traits deal with violated expectations, where user experience and intuition are not served by the implementations of geometric design, TCDs, or operations and maintenance. These traits are typically contributing and are caused by categories, traits, and attributes of groups 3000, 4000, 5000, and 6000 and causative of the traits and attributes of categories 1300 and 1400.

The user profile traits of category 2200 (see table 12) address user characteristics that have been demonstrated to be associated with driver performance and may exacerbate the driver’s response to complexity. Of particular interest here is trait 2240, driver age, as FHWA’s ongoing efforts to address the needs of the aging driver population have identified needs that older drivers have, particularly related to challenging and unfamiliar driving environments.⁽²⁹⁾

A brief overview of commercial motor vehicle (CMV) operations indicated specific traits that may present challenges for CMV operators (see table 13). While additional traits related to user characteristics could be assigned to category 2300, such work should involve the Federal Motor Carrier Safety Administration and is outside the scope of this report.

Environmental characteristics have a definite impact on users. Category 2400 (see table 14) addresses environmental characteristics and includes some traits related to category 1100 attributes and traffic flow as causative of HFs responses rather than indicative of congestion.

Group 3000—System Design

The group 3000 categories, traits, and attributes address system design. System design is the overall layout of the road network, including route marking, and the overall design of interchanges, including interchange configuration. System design does not address geometric design choices (e.g., ramp terminal spacing and ramp terminal design). The group 3000 attributes generally contribute to the complexity of the navigation task.

Category 3100 (see table 15) addresses interchange configuration, a component and expression of category 3200 traits and attributes (see table 16), which address system configuration. Interchange configuration traits and attributes include the types and presence of movements, and the application of these attributes influence how road users form a mental image of the interchange and its layout. While geometric design is addressed in group 4000, the overall alignment of the interchange is often a function of its configuration, and alignment-related attributes are addressed in trait 3150.

Trait 3230 addresses surface street interactions, which can aid or hinder the navigation task on surface streets and contribute to crashes that involve vulnerable users. Additional research on crashes related to surface-network complexity concerns interchange access points, where drivers expect higher speeds and a reduction in delay, which will help develop relationships between network navigation task workload and driver attentiveness to vulnerable users.

Subsequent to the collection and analysis of transportation planning data, the process of designing system interchanges considers the selection of an interchange type based on the basic configurations in the AASHTO Green Book.⁽¹⁶⁾ The selection of the interchange type is often predicated on the typical configurations within the corridor, the spacing of ramp terminals, the need for specific operational strategies on surface streets, and agency experience with constructing and operating particular interchange types. As the design develops, practitioners make choices concerning geometric design, including the location and spacing of ramp terminals; the provision of movements within an interchange network; the geometric design characteristics of the ramps and intersections; and the means of providing for guide signing, route marking, and wayfinding within interchanges.

Existing interchanges may contain less-than-optimal geometric design characteristics and may be inadequate to support demand. In some cases, interchanges were designed and built to provide for some future higher-order interchange or access, and the characteristics of such an interchange may lead to motorist expectations that are at odds with the interchange’s function in the network. In other cases, even the choice of arrow type on an overhead sign may be misleading for the design speed of movements or even the location and arrangement of turn lanes.

The six-ramp partial cloverleaf, for example, can be configured in two ways. In figure 5, the interchange shown permits right turns from the surface roadway and a single exit from the mainline. An alternate configuration places the loop ramps as departures from the mainline or a continuous or terminating C/D roadway. This configuration can result in limited visibility of pedestrian crossings on the loop ramps at the surface street. It also necessitates the provision of either an exit with a downstream split or two exits from the mainline for the intersecting roadway. Attribute 3132 could be a contributing factor in intersection crashes in service interchanges where the roadway geometric design appears to support a higher-order interchange.

©The American Association of State Highway and Transportation Officials. Used with permission.

Figure 5. Graphic. AASHTO Green Book excerpt of partial cloverleaf design A, depiction E from figure 10-1.

Attribute 3142 occurs in both urban and rural settings. Practices for signing indicate there is no reentry to the motorway. In some cases, the non-provided movements occur in system interchanges between major routes. The lack of a connecting movement may lead to road-user misrouting, erratic lane changes, and general system inefficiencies. The provision of adequate advance signing is essential. Attribute 3222 is evident in the downtown core areas of several large cities, including on eastbound I-94 in Saint Paul, MN.

Group 4000—Roadway Geometric Design

Group 4000 addresses the geometric design of the roadways within complex interchanges. Category 4100 includes attributes for lane configurations on the approaches to exits and within the ramp terminal areas, including lane balance characteristics. The group 4000 categories, traits, and attributes contribute to the complexity of the guidance task, including lane selection and time-based demands on drivers. While these are influenced by interchange layout, they are factors generally related to geometric design choices.

Category 4100 is divided into four traits (see table 17 for traits 4110 and 4120; see table 18 for traits 4130 and 4140). Trait 4110 relates to the length and presence of auxiliary lanes, as defined by AASHTO, between interchange segments and in advance of exit ramps and subsequent to entrance ramps. Trait 4120 addresses the roadway design characteristics and cross section upstream of and at exit ramp terminals. Trait 4130 addresses entering lanes and the geometric design characteristics of cross-section and acceleration lanes. Finally, trait 4140 relates to the concept of lane balance, which is evaluated for all ramp terminals. Many of the attributes are merely characteristic, but some can be both mitigating and contributing. For example, attribute 4136 may appear to be mitigating, as long acceleration lanes might help to reduce weaving and other unsafe driving behavior, but improper or insufficient signing (addressed from group 5000) of such situations can impact traffic safety and operations because drivers may become confused. Warning sign installations should be considered in these cases.

Trait 4140, lane balance, includes one type of lane balance addressed specifically by AASHTO’s Green Book.⁽¹⁶⁾ The design of entrance ramps for freeway facilities is typically of two types, either the parallel design or the tapered design. Either design is acceptable for single-lane entrances, but the use of the tapered design for multilane entrances can create complications. In situations when the tapered design is used for multilane entrances at major convergences, there is a high potential for safety and operational drawbacks, particularly with large vehicles. Road-user operation, in this environment, can be especially demanding because both the operation and piloting tasks are taxed as users anticipate and execute the merging maneuver. The following list identifies the impacts of the tapered design characteristics for multilane entrance ramps:

• No clear assignment of right-of-way exists for entering vehicles. Although vehicle paths and the through lane could be remedied with pavement marking, it would remain unclear how the yielding vehicle is to avoid a conflict.
• No recovery area is available for vehicles that are unable to complete or are prevented from completing the merge. The lack of a shoulder for either merging vehicle eliminates an escape path.
• Increasing taper distances compound uncertainty. As the volumes and design speed increase, the longer taper creates a longer area with enough width for two lanes, further reducing the intuitive visual impact of converging pavement markings.
• Larger heavy vehicle volumes can increase difficulty of merging maneuvers. Trucks need more room to negotiate the merge and space to occupy the destination lane. This can precipitate lane changes and speed reductions in the destination lane, impacting the capacity of the affected lane and increasing safety risks.
• Capacity collapses under high volumes. The merge maneuver required by the tapered design requires more judgment, a difficult speed-matching activity, and more time, all of which are factors leading to a potential reduction in capacity. The parallel design is generally preferred because it enables drivers to perform a simple lane-change maneuver from the adjacent and parallel lane.

Furthermore, FHWA has published the Highway Design Handbook for Older Drivers, often referred to as the Design Handbook, which cites research that indicates tapered merges, even for single-lane entrance ramps, are difficult to navigate.⁽²⁹⁾ Add to that all the insufficiencies of the inside-lane merge situation, and the case could easily be made against such installations in high-volume system interchanges. Principles of design for older drivers are applicable to most geometric design issues and certainly a worthwhile study in any design undertaking. The Highway Design Handbook for Older Drivers states the following:

Another issue addressed by NCHRP 3-35 was acceleration lane geometry. Koepke (1993) reported that 34 of the 45 States responding to a survey conducted as a part of NCHRP 3-35 on SCL’s use a parallel design for entrance ramps. Thirty of the agencies interviewed use a taper design for exit ramps and a parallel design for entrance ramps. The parallel design requires a reverse-curve maneuver when merging or diverging, but provides the driver with the ability to obtain a full view of following traffic using the side and rearview mirrors (Koepke, 1993). Although the taper design reduces the amount of driver steering control and fits the direct path preferred by most drivers on EXIT ramps, the taper design used on entrance ramps requires multitask performance, as the driver shifts between accelerating, searching for an acceptable gap, and steering along the lane. Reilly et al. (1989) pointed out that the taper design for entrance lanes poses an inherent difficulty for the driver and is associated with more frequent forced merges than the parallel design. Forced merges were defined as any merge that resulted in the braking of lagging vehicles in Lane 1, or relatively quick lane changes by lagging vehicles from Lane 1 to a lane to the left. The parallel design would thus appear to offer strong advantages in the accommodation of older driver diminished capabilities. (p. 137)⁽²⁹⁾

The Green Book addresses the issue of the parallel design in chapter 10: “Generally, parallel designs are preferred. While tapered designs are acceptable, some agencies are concerned about the inside merge on the tapered entrance ramps” (p. 821).⁽¹⁶⁾ The project team has identified approximately 12 sites in Illinois, including 1 constructed as recently as 2006. One site, the convergence of eastbound I-80 and I-94 in Lansing, IL, features a multilane entrance of the tapered design where the minimum recommended merge length of 2,500 ft was not met (see figure 6). Both freeways feature a truck percentage of approximately 50 percent.⁽³⁰⁾

©Esri.

Figure 6. Photo. Multilane entrance ramp with tapered design, I-80 at I-94 eastbound, Lansing, IL.⁽³¹⁾

Category 4200 includes traits and attributes that address ramp terminal arrangements, including the spacing and relative arrangement of ramp terminals (see table 19). Ramp terminal arrangements are independent of lane configuration. The key trait in this category is trait 4260: decision point interactions (see table 21), which is a candidate for future research examining the effect of ramp terminal arrangement choices, relating in particular to multiple categories in groups 3000 and 4000.

Traits 4220 and 4230 are omitted from the current version of the attributes list to permit future expansion of the attribute tables to address other interactions related to geometric design considerations considered in ramp terminal design, particularly related to managed lanes and tolled facilities.

Trait 4240 (see table 20) addresses the ramp terminal arrangements from the perspective of ramp sequence and road-user perception of exit order and proximity. Specific information that explains some attributes associated with trait 4240 is provided in the subsections that follow.

Attribute 4244—Exit with Downstream Left Exit from Distributor Roadway

Left exits present challenges from mainline freeway lanes and can lead to confusion on distributor roadways. In the case shown in figure 7, the left exit from the distributor roadway for I-4 southbound carries left-turning traffic to United States Route (US) 192 eastbound. However, the primary movement in the interchange is the ramp to US 192 westbound, which is accommodated with two lanes.

©Esri.

Figure 7. Photo. I-4 distributor roadway upstream of US 192 interchange in Kissimmee, FL, showing left exit from the distributor roadway.⁽³²⁾

In this particular case, upstream signing is provided, advising road users who intend to head eastbound on US 192 “TO KEEP LEFT.” This type of signing is not uniformly provided in similar circumstances, however, and additional emphasis on the presence of a left-hand movement, such as a “LEFT EXIT” supplemental plaque, is typically warranted when high volumes are present.

An example of explicit, simplified signing, sometimes referred to as “positive guidance,” is illustrated in figure 8, where multiple signs are provided, including a ground-mounted exit-direction sign and overhead exit-direction sign with angled down arrows.

Source: FHWA.

Figure 8. Photo. Ramp from I-35E southbound to I-94 and US 52, Saint Paul, MN.

Attribute 4245—Directional Ramps Out of Order Relative to Direction of Travel

One example of attribute 4245 is the interchange of US 175 and I-20 southeast of Dallas, TX (see figure 9). The southeast-bound movements from US 175 to I-20 are out of order relative to conventional thinking about left and right turns.

©Esri.

Figure 9. Photo. US 175 interchange with I-20 in Dallas, TX, with upstream exit carrying left-turning traffic to northeast-bound I-20.⁽³³⁾

Typically, to turn right, a road user would keep to the right and, therefore, be making the first right-hand movement. In some applications of this interchange configuration, turning right requires remaining out of the right-hand lane and using the second right-hand exit. On this approach, the right lane serves the left-turning movement first and then terminates as a mandatory movement lane to the lower-volume movement to westbound I-20. This case compounds the problem as it also involves topic 3, the upstream exit from a lane terminating as a downstream exit-only lane.

In contrast to trait 4240, trait 4250 addresses the configuration of the interchange in localized areas related to how ramps are positioned relative to the roadway (see table 21). While ramp sequence is the focus of trait 4240, ramp configuration and access are the focus of trait 4250.

Attribute 4253 (“swap-sided” exits), while occurring rarely, is particularly problematic. When directional exits are available from both sides of the freeway, road users typically expect that the left-side exit will provide a left-hand movement and the right-side exit a right-hand movement. However, if the exits are swapped, not only does the left exit exist but it is also not intuitively directional.

Such circumstances exist when the right-hand movement is the primary direction of travel and carries a marked route associated with the upstream segment. In these cases, additional signing and explicit use of geographic destinations are often provided, in addition to posting of the marked route and direction in conjunction with the exit gore sign.

Category 4300 traits and attributes relate to the cross-sectional elements of geometric design (see table 22). Numerous research studies have indicated that these elements alone do not contribute to complexity but can exacerbate the effects of complexity and contribute to crashes where other attributes exist. In particular, the lack of shoulders (attribute 4311), when associated with attribute 4142 (the tapered multilane merge), eliminates a potential escape path for vehicles that cannot merge as a result of lack of a gap or driver hesitancy. In addition, attribute 4142 was found in this research to be typically associated with traits 2120, 2130, 4340, and 4350.

Group 5000—TCD

TCDs provide information that aids both the navigation and guidance tasks and, when implemented consistently and when needed, can reduce the complexity of the navigation task considerably.

Category 5100 traits and attributes, which deal with traffic signing, address the mitigating and contributing attributes with the recognition that some of those attributes, when presented in a physical device, could reduce complexity or increase it.

Traits 5110 and 5120 address information load in both proclivity (e.g., the amount of information and its distribution) and message characteristics (e.g., the composition, configuration, and type of messaging) (see table 23). One example of this is attribute 5122, use of route names with route shields. In Chicago, IL, and in the New York metropolitan area, for example, these names are used in common parlance and are an aid to the navigation task. In other areas, the superfluous information on guide signs may simply mean additional information is being presented for processing, which increases driver workload.

Trait 5130 attributes address the design and implementation of guide signing for option lanes (see table 24). The MUTCD currently provides four methods for signing option lanes, and States are using additional methods, some uniformly and others not appearing to adhere to existing practices or sign design principles.

For the purposes of this research, three styles of option lane signing were considered. The method used most often is the discrete arrow method, where a single down arrow is provided over each lane in advance of the interchange, typically with signing over only the exiting lanes. The newly introduced APL method is described here as the blended arrow method, because it uses a combination of arrows on the sign panel, some with both one arrowhead and others with two. Finally, the venerable diagrammatic method is considered. This report does not address the problems associated with multiple down arrows pointing into a single lane.

Trait 5140 attributes describe guidance for freeway signing in the gore area (table 25).

The attributes of trait 5190 (see table 26) are considered pivotal to the design of good overhead freeway signing. The use of guide sign-specific arrows from the MUTCD ensures arrow legibility from a distance. Arrow type, size, angle of rotation, and position on the sign all combine to convey specific information from distances beyond the legibility distance for associated word messages and other symbols.

Roadway delineation, in the form of pavement markings and roadside delineation, is critical to the guidance task and must support the navigation task. Category 5200 traits and attributes address pavement marking by marking orientation in three traits: longitudinal, transverse, and symbols (see table 27). In addition, the use of supplemental and substitute markings, including raised reflective pavement markers (RRPMs), is imperative for lane delineation during inclement weather, and roadside delineation is used in States where snowfall is experienced to aid in the guidance task and to assist with maintenance operations.

Group 6000—Management and Operations

System management and operations philosophy, practice, and execution continue to have an increasing impact on traffic operations. The implementation of these strategies, though often a mitigating factor in traffic congestion, can contribute to interchange complexity. Nearly all of these attributes result in an increased driver workload in advance of decision points and generally require driver knowledge of the management strategy to guarantee comprehension and proper use.

Category 6100 traits and attributes relate to restricted and managed facilities, including restricted-use lanes, managed lanes, tolled lanes (addressed specifically in category 6500), and various iterations of the implementations that can include reversible facilities and other management strategies designed to optimize the use of the network (see table 28).

Category 6200 includes system management strategies that microscopically manage demand, as opposed to the categories 6100 and 6500 attributes, which are macroscopic-level demand-management tools (see table 29). While certainly more tools are available, the project team identified one trait that can increase the complexity of the navigation task, particularly when longer queues are involved.

The information systems traits and attributes in category 6300, like most system management strategies, can mitigate complexity by aiding in the navigation task but can also increase complexity where information processing workloads are highest (see table 30). Understanding driver reaction to user information and the sequencing and presentation of user information is the key to reducing the adverse effects of user-information systems on driver workload in complex environments.

Category 6400 traits and attributes relate to active traffic-management systems (see table 31).

In practice today, no system management strategy seems to be evolving faster than roadway pricing, either for congestion management or simply for finance and operations (see table 32). Congestion pricing by means of road segment pricing is conducted by a variety of schemes and, even within one region, various schemes are applied to the roadway network or even along the length of the corridor. Road-user decisionmaking, particularly related to navigation and lane selection, can become a task-saturated process when road users must make decisions on price tolerance, compliance with regulations, and destination availability from the managed facility in a framework that changes throughout a region.

Category 6600 traits and attributes relate to incident response and resilience, including the operation and work of traffic-management centers (TMCs) and incident management strategy (see table 33). In an interchange with other attributes related to complexity, incident response operations can exacerbate complexity by increasing driver workload, reducing capacity, and affecting the spacing between access points.

As system management strategies and techniques further evolve, it will likely be necessary to divide group 6000 into additional groups. Future publications concerning freeway operations, congestion pricing, and incident response will guide this work. To accommodate this, the project team left group 7000 and group 8000 unused.

Group 9000—Institutional Factors

The categories in group 9000 relate to institutional factors, typically exclusive of technical policy addressing the implementation of TCDs (see table 34). Agency policies, processes, and preferences for planning, design, design documentation, standards development, and cost control all affect the design and operation of interchanges in urban areas. Overall agency philosophy, particularly as it relates to the importance of traffic engineering support and HFs integration, is cultivated over a length of time, and long-term philosophies can also affect the agency’s delivery of projects, operations, and maintenance. Finally, fewer agencies today plan for long-term facility expansion, but those that do tend to integrate future geometry into existing projects, ensuring that future constructed improvements satisfy geometric design requirements.

Selection of Topics

Attributes from all groups were examined so that related attributes and those with potential and known interactions could be organized and addressed with a single research activity. The following criteria were used to help identify topic areas and select attributes related to those topic areas:

• Crash modification factor for improvements rates higher than other attributes.
• Demonstrated to affect capacity and or safety performance.
• Attribute common in many older or poorly performing interchanges.
• Subject to correction with TCD changes and minor geometric adjustments (for existing interchanges).
• Identified as critical or problematic by the stakeholder working group.
• Identified as recurring design flaw/shortcoming by the stakeholder working group and the project team.
• Determined to be an area of inconsistent practice in field applications.

Table 35 lists the 10 topics used to develop the research plan in addition to identifying the type of testing proposed to address each topic. These testing types generally fell under the work of the simulator study (chapter 6) and the field study (chapter 7), which were designed to develop a better understanding of these attributes and their interactions.