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

Chapter 8. Key Findings

In chapter 1 of this report, the basis of the following recommendations is outlined. Each of these research activities (data collection, analysis, and interpretation) generated results. Based on those results, the project team developed key findings, which are used to prepare the recommendations in chapter 9.

Practitioner Input

The practitioner perspective was obtained from interviews with participants of the working group, from National Committee on Uniform Traffic Control Devices members, and from a topical search of webinars provided by the ITE and similar organizations.

Practitioners shared ideas on staffing, policy initiatives, quality management, and sign fabrication in contract delivery. In general, agencies prefer central office management of sign design practices and policy, but personnel without significant training and experience in freeway signing are often appointed to positions that demand HFs expertise. There is increased desire to provide training for distributed staff and to ensure that traffic engineering personnel have contract oversight in the project delivery process.

Literature and Technical Policy Review

The literature and policy review found that some gaps exist in technical literature related to specific sign design standards. In addition, many agencies do not publish a large-format sign design manual, typically intended for guide signing on freeways, expressways, and primary highways, and the MUTCD and SHS are the sole source of large-format sign design policy for agencies.

MnDOT publishes a Traffic Guide Sign Design Manual (most recently published in 2015) that includes substantial information about specific design practices (e.g., fraction layout, legend arrangements, and arrow selection and use).⁽³⁴⁾ WSDOT also publishes specific practices related to large-format signing in chapter 2 and the appendix of the WSDOT’s Traffic Manual (publication M55-02).⁽⁵¹⁾ The practices evaluation revealed that States with a history of deliberate policy development generally exhibited fewer sign errors in field installations and that signing approaching interchanges was more consistent between geographic regions and among signing of various agencies.

Practices Evaluation

The outcome of the practices evaluation was a summary of practices in partnership States and throughout the United States. In general, these practices were identified as being related to attributes contributing to complexity and were evaluated on the basis of conformance to the consistency principle, which was introduced in chapter 5.

The practices evaluation identified nine areas where conformance to the consistency principle was either uniformly observed or was not observed:

• Option lane signing, including policy in the MUTCD.
• Placement of exit-direction signs.
• Legend arrangement on sign panels.
• Use of guide sign arrows.
• Use of distances on all primary advance guide signs.
• Separation of sign panels for separate movements.
• Differentiated use of broken lane lines, dotted lane lines, and dotted extension lines; use of solid lane lines for lane separation and within exit ramp terminal areas.
• Delineation of exit ramp terminal areas.
• Signing of lane reductions, for both entering lanes and continuing lanes terminating at a downstream location.

While many of these practice areas have been addressed in the literature, some existing practices that were observed as highly correlated with consistency have been justified on the basis of heuristics. For example, some States use only one arrow per lane while others mix the use of arrows depending on regional preferences. The consistency principle provides the logical framework for evaluating the use of these TCDs and indicating where evaluation of the effectiveness may be warranted.

Simulator Study

The simulator study analysis examined lane choice selection accuracy, ULCs, and the distance over which a subject traveled prior to making the final lane selection for a given simulator scenario. The results of the analyses indicated that the performance of subjects for most scenarios was statistically indistinguishable.

Two variables indicated statistically significant deviation. The first was the performance of subjects in terms of ULCs for signing alternatives in layout A. The second was the performance of subjects in terms of “survival,” or upstream final LSD, for signing alternatives in layout E.

Layout A

The data analysis for layout A indicated that subjects made fewer ULCs in signing alternatives A1 and A2 than subjects in signing alternative A3. The analysis also indicated that subjects in alternative A1 made final lane selections further upstream than those subjects in signing alternatives A2 and A3. By both metrics, the simulator study results suggest that signing alternative A1 exhibits better performance in terms of securing early final lane selection and limiting lane changes.

While signing alternatives A2 and A3 do uphold the principle of separated sign panels for separate movements, it is clear that, in this laboratory test, the signing for alternative A1 was associated with better performance for increasing early confidence in lane selection. Minor changes to the design of the signs in alternative A1, which were created to duplicate field conditions, will maintain the single-sign-panel/single-arrow approach while clearly indicating that there are two subsequent exits. The designs in signing alternative A1 already include distances to the exits and, in addition, separating out the exit numbers into the two separate segments of the sign panel will also clarify that there are two separate exits, even though a single panel is used.

Layout E

The data analysis indicated that subjects made a final lane selection in signing alternative E3 nearly twice as far away from the mainline exit as subjects in signing alternatives E1 and E2. Signing alternative E3 differs from signing alternatives E1 and E2 in that it provides separate sign panels for each exiting lane, including the option lane. The left lane is signed with the destination of both lanes of the distributor ramp, and the right lane is signed with the exiting destination for the downstream exit.

In layout E, each of the two exiting lanes in the distributor roadway is assigned discretely to a downstream exit; the left lane is a dedicated lane for the left-hand movement, and the right lane is a dedicated lane for the right-hand movement. This means that signing alternative C3 is directly applicable to the geometric design in the interchange layout and does not violate motorist expectation. In layout E, however, both exiting lanes are dedicated to the left-hand movement, and the right-hand movement is provided by means of a single exiting lane from the right-hand lane of the distributor roadway.

The signing in alternative E3 does not, therefore, adhere to the consistency principle. This is an example of broadening application, where the functional case that is applicable to layout C is applied to layout E, despite the difference in geometric design. Alternative E3 does not adhere to the consistency principle for the configuration of the downstream distributor roadway. Moreover, alternative E3 does not explicitly indicate to road users that both lanes are available for the left-hand movement, which could have significant impacts on traffic operations, including the potential starvation of the right lane and problems related to road-user inability to access the right lane because of congestion in the second lane from the right.

Field Study

The field study explored driver behavior at complex interchanges in several areas across the country. One common finding across sites is that exiting traffic typically used the exit-only lane rather than the option lane. The cluster analysis found only one site where the most common group of driver behavior did not include moving to the exit-only lane before, or near, the first guide sign with a yellow exit-only panel. The analysis also identified examples where drivers would use the exit-only lane and, ultimately, make a lane change on the distributor roadway. These instances include signing that does not indicate the presence of an option (e.g., eastbound I-4, exit 67) lane or groups several destinations on a single sign (e.g., southbound I-5 at exits 105 B-A). Conversely, the cluster analysis did not find any instances of common behavior where drivers exited using the option lane and then changed lanes on the distributor roadway.

The site with the highest option lane use was found to be Washington State site 43-2, southbound I-5 approaching the US 101 exit. At this site, nearly half of the drivers were observed using the option lane. This site also included a guide sign configuration not seen in any of the other sites. On the advance guide sign, the option lane is signed with the standard downward white arrow on a green background; however, the guide sign also includes the letters “OK” adjacent to the downward arrow.

In the Atlanta, GA, site, approximately 12 percent of the drivers exiting at the second exit traveled in the exit-only lane for the first exit before changing out of that lane near the exit ramp for the first exit, ultimately using exit 51B. This behavior occurred despite the presence of “EXIT ONLY” signing over the lane for the first exit.

Summary of Key Findings

Overall, the project team’s efforts in the various activities led to the following six key findings (many of which align with the strategic emphasis areas of FHWA’s Office of Safety, particularly roadway departure crashes):

• Consistent application of signing principles, both among locations and within various geometric design scenarios, leads to correct driver response.
• 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.

Option lane signing may take multiple forms, but the basic concepts of signing for the option lane and differentiating between upstream and point-of-departure signs should be incorporated into option lane signing policy. The overall finding of this research is that consistency in TCD applications is the key principle in facilitating road-user navigation and guidance tasks within complex roadway environments, including interchanges. Signing, pavement markings, and geometric design should be applied consistently among interchanges, and even within the components of a system, to support road-user expectancy. Consistent application of TCDs, including the discrete and differing treatment of various configurations along the approach to an interchange, ensures road-user expectancy is not violated.