Median Intersection Design for Rural High-Speed Divided Highways (2010)

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147 Conclusions A rural expressway is a high-speed, multilane, divided high- way with partial access control that may consist of both at-grade intersections and grade-separated interchanges. Many states are converting rural two-lane undivided highways into expressways for improved safety and mobility, but at-grade intersection col- lisions on rural expressways are reducing the safety benefits that should be achieved through conversion. Right-angle collisions (particularly on the far-side) are the predominant crash type at conventional TWSC rural expressway intersections. The underlying cause of these collisions in most cases is the inabil- ity of the driver stopped on the minor road approach to judge the arrival time of approaching expressway traffic, so assisting minor road drivers with gap selection is crucial to improving safety at TWSC rural expressway intersections. Currently, there is a shortage of design options in the AASHTO Green Book (3) and the MUTCD (22) that address the issue of gap selection; therefore, the primary objective of this research project was to suggest improvements to the available design guidance in those manuals for TWSC rural expressway intersections. Traditionally, when the safety performance of these inter- sections begins to deteriorate, the countermeasure application path illustrated in Figure 117A starts with several low-cost signing, marking, or lighting improvements; followed by sig- nalization; and ultimately grade separation. However, high- way designers need other options because the high cost of interchanges limits their use on expressways, and TWSC rural expressway intersections often experience safety problems long before traffic signal volume warrants are met. In addition, sig- nals hamper the mobility expressways are meant to provide, and they don’t always improve safety as intended. The case studies presented in Chapter 4 of this report reveal that there are promising safety treatment options for TWSC rural ex- pressway intersections that address the gap selection issue while avoiding signalization and grade separation. These case studies help us begin to understand the safety improvement potential of these countermeasures and set the stage for the de- velopment of a richer set of design options as shown in Figure 117B. However, because sufficient sample sizes were not avail- able for any of the case study treatments, future research is nec- essary to determine accurately the safety effectiveness of these non-traditional designs as well as the conditions under which each countermeasure should be considered and under which each one would be expected to fail in terms of safety and/or op- erations. Aside from recommending improvements to the Green Book and the MUTCD, only more experimentation with the non-traditional designs can be recommended. Although this report identifies many issues, most must be solved by others in the future. For example, although thorough reviews of the Green Book and the MUTCD were conducted with many resulting recommendations, modifications to the Green Book are made through the AASHTO Technical Committee on Geometric Design and changes to the MUTCD are made through FHWA’s MUTCD Team and the National Committee on Uniform Traffic Control Devices. The recom- mendations provided are meant for the consideration of these groups, and it is ultimately their responsibility to actually modify the contents of those manuals. Furthermore, the safety effectiveness of the rural expressway intersection treatments examined in the case studies can only be determined if STAs are willing to deploy and to evaluate them rigorously. While the recommendations that follow are specific, others must implement them to positively impact rural expressway inter- section design and safety. Recommendations Recommendations for Design Guidance and the AASHTO Green Book A thorough evaluation of the design guidance for TWSC rural expressway intersections contained within the 2004 AASHTO Green Book (3) was conducted (see Chapter 2 and Appendix A) in an attempt to identify areas where the existing guidance might be lacking. For example, the Green Book de- C H A P T E R 5 Conclusions, Recommendations, and Future Research Needs

scribes dimensions for turn lanes, corner radii, and median noses, but no indication is given as to how these features con- tribute to safety. On the other hand, there is no design guid- ance describing how to restrict median access, but there is a great deal of information relating access and safety. As a result, limitations were identified and recommendations for poten- tial Green Book revision were separated into three general cat- egories: organizational changes, philosophical changes, and design guidance updates. The suggested changes within these three areas are summarized in Table 16 and described here. Organizational Changes In the 2004 AASHTO Green Book (3), design guidance for rural expressways and their intersections is spread throughout several chapters, which may create confusion for roadway designers. This is probably due to the fact that expressways are a hybrid with some elements designed like freeways, while other elements (particularly intersections) are designed similar to rural undivided highways. An ideal solution to this problem would be to reorganize the Green Book so that all material on rural expressways and rural expressway intersections is included in a single comprehensive chapter as has been done for freeways (Green Book Chapter 8). However, members of the AASHTO Technical Committee on Geometric Design have expressed concern over this reorganization, noting that it would be a tremendous undertaking and that the modifications might not address all of the issues while potentially creating other confusion in using what is already a cumbersome guide and reference manual. An alternative reorganization strategy may be to revise Chapter 9 of the Green Book to include a separate section on expressway intersection design. A final 148 Figure 117. TWSC rural expressway intersection countermeasure matrices.

option and a more realistic approach may be to create a sep- arate complementary manual for expressway design similar to ITE’s Freeway and Interchange Geometric Design Handbook (24). Because expressways do not have a rich history of guid- ance and literature like freeways, the first edition may only address the design issues identified in this document and map out future research needs, but once this “expressway handbook” becomes mature, the most essential information it contains could be incorporated into the Green Book. Philosophical Changes Rural expressway corridors typically outlast their at-grade intersections in terms of both safety and operational efficiency as highway designers are usually unable to design an express- way intersection to be both safe and efficient for more than 10 years. TWSC rural expressway intersections tend to experi- ence safety issues long before they experience congestion. When the safety performance of an at-grade expressway inter- section begins to deteriorate, countermeasures are considered at that time. This current philosophy is reactive and problem- atic as the countermeasures may take years to develop while the safety issues continue to occur. Although Chapter 7 of the Green Book contains a brief discussion regarding planning for the ultimate development of four-lane divided rural arterials, it does not address planning for specific intersection modifi- cations that may be required before the end of the design or functional life of an expressway corridor. Therefore, the Green Book should eventually include a more proactive expressway intersection safety planning process with triggers defining when to start planning for or constructing the next level of intersection design as a conventional TWSC intersection tran- sitions into a full interchange over the course of its life cycle. In addition, expressway intersection safety should be more ac- tively considered during the initial expressway corridor plan- ning and development process through the strategic place- ment of intersections on tangent sections; the reduction of intersection skew; and improved access control through the use of frontage roads, offset T-intersections, and J-turn inter- sections along the corridor. Design Guidance Updates Perhaps the most important recommended update to the Green Book is to include design guidance for the rural express- way intersection designs that eliminate or reduce far-side con- flict points (J-turn intersections and offset T-intersections) or those that address the issue of gap selection for minor road drivers (median acceleration lanes and offset right-turn lanes). Within Chapter 9, the current edition does a good job describing the design of traditional four-leg and three-leg stop-controlled rural divided highway intersections. However, the far-side right-angle crash problem and the associated minor road driver gap selection issues are not discussed in relation to these intersections. Furthermore, when these tra- ditional designs begin to experience safety and/or operational problems, roadway designers are only given design guidance for a few corrective intersection alternatives (offset left-turn lanes, indirect left-turns via jughandles or median U-turns, and interchanges). No design guidance is currently avail- able in the Green Book regarding J-turn intersections, offset T-intersections, median acceleration lanes, or offset right-turn lanes. Consequently, few STAs are using these designs. In addition, the existing design guidance for jughandle inter- sections, median U-turn intersections, and offset left-turn lanes is extremely limited and should be updated to reflect current STA practice. The Green Book should eventually include design guidance and guidelines addressing the conditions under which each design should be implemented to more fully develop the countermeasure matrix shown in Figure 117B. Recommendations for Design Guidance and the MUTCD A thorough evaluation of the 2003 edition of the MUTCD (22) was conducted (see Chapter 2) regarding the signing, marking, and traffic-control devices used at TWSC rural expressway intersections to identify areas where the existing guidance might be insufficient. Limitations were identified and suggested opportunities for revision were separated into three general categories: assistance for minor road drivers, assistance for expressway drivers, and other technical modi- fications. The recommendations within these three areas are summarized in Table 17 and briefly discussed here. Assistance for Minor Road Drivers Currently, the MUTCD identifies a number of signs and markings that are intended to help a minor road driver recog- nize an approaching stop-controlled intersection (over-sized signs, advance warning signs, pavement messages, flashing beacons, etc.). However, there is no mention that intersection recognition is less of a contributing factor in intersection crashes than gap recognition and selection, for which no devices/driver aids are identified. Therefore, a primary enhancement to the current MUTCD (22) guidance for TWSC rural expressway intersections would be to identify and incorporate any traffic- control devices or markings to assist minor road drivers with their decisionmaking processes for judging and selecting safe gaps in the expressway traffic stream. Currently, the MUTCD does not address the need for or the application of such de- vices and/or markings. Even though there is no widely ac- cepted device to assist with gap selection from the minor road, there have been attempts to develop and deploy experimental systems such as IDS technology, static roadside markers, me- dian pavement markings, and median signage. These devices 149

are meant to inform minor road drivers of the size and availability of gaps in expressway traffic, to encourage a two-stage gap selection process, and/or to remind drivers to look both ways again before proceeding. The MUTCD should provide some guidance and uniformity for the use of such devices as experimental treatments or after they have been sufficiently proven to be effective gap selection aids. Assistance for Expressway Drivers Another enhancement to the current MUTCD would be to include language supporting the use of intersection recognition signing strategies on the expressway approaches (i.e., freeway- style guide signs, diagrammatic guide signs, dynamic warning signs and flashers, or other such devices) to help expressway drivers identify TWSC intersections with a higher crash risk so that they might apply extra caution when approaching these intersections. As pointed out throughout this report, not all TWSC expressway intersections have the same crash risk. The relative safety of an intersection depends on many factors, but skewed intersections, intersections where the mainline is on a horizontal or vertical curve, intersections with high minor road volumes, intersections with extreme hourly peaking on the minor road, or intersections with some combination of the above tend to have higher crash frequencies/rates. These characteristics seem to make it more difficult for minor road drivers to select safe gaps. Although this strategy would not aid minor road drivers directly in this regard, it would alert the expressway driver to the increased potential for conflict so that they might be prepared to take evasive action as necessary should a minor road driver select an unsafe gap. Existing MUTCD guidance in Section 2E.26 calls for similar guide signs to be used at expressway intersections as on conven- tional roadway intersections with an option for diagrammatic guide signs to be used at expressway intersections, but no exam- ples of diagrammatic guide signage for at-grade expressway intersections are provided. It is recommended that MUTCD Section 2E.26 provide more specific examples and guidance for when freeway-style guide signs with diagrammatic layouts are appropriate. In addition, the use and application of dynamic mainline warning devices—VEHICLES ENTERING WHEN FLASHING or WATCH FOR ENTERING TRAFFIC signs with flashers—are not described and no guidance as to when a high- way agency should consider this type of advance intersection warning is given. Therefore, future MUTCD editions should include mainline intersection recognition strategies that alert expressway drivers to the presence of high-risk intersections. Other Technical Modifications A number of technical modifications to the current MUTCD (22) guidance are suggested. These modifications are listed in Table 17 and are possible without any further research or development. These technical modifications can be grouped into three areas: figure modifications, figure additions, and addressing inconsistencies between the MUTCD and the Green Book. The figure modifications involve changes to MUTCD Figures 2B-10, 2B-13, and 2B-15 (see Figures 22, 19, and 21, respectively). The recommended figure additions include adding figures for • Standard signing and marking at a conventional TWSC expressway intersection with a median width of 30 ft or more and offset left-turn lanes; • Wrong-way signing for a conventional TWSC expressway intersection with a median less than 30-ft-wide; • Warning and/or guide signing for conventional TWSC expressway intersections and those with elevated crash risk, and • Standard signing and marking for the non-traditional expressway intersection designs (i.e., J-turn intersection, offset T-intersection, jughandle intersections, median accel- eration lanes, and offset right-turn lanes). Signing standards for these non-traditional expressway intersection designs are necessary for nationwide uniformity and should be based on the experience of STAs that have already experimented with these designs and developed their own standard signing plans. Finally, inconsistencies in the median width definition and the minimum median storage requirements need to be cleared up between the MUTCD and the Green Book. Future Research Needs Chapter 4 of this report contains 10 case studies of inno- vative TWSC rural expressway intersection treatments that are believed to improve intersection safety. Other treatments (such as the single-quadrant interchange) could also have been included as case studies, but were not ranked as highly by the project panel. Naïve before-after crash data compar- isons were performed for 7 of the 10 treatments examined (no data were available for the other 3) and most illustrated improved overall safety and/or a reduction in the targeted crash types. However, a limited number of sites were examined and, in some cases, the amount of before and after crash data were inadequate to perform any statistical evaluation. Further- more, the naïve before-after analysis methodology does not take regression-to-the-mean into account, and it is not known exactly what part of the noted change in safety can be attrib- uted to the treatment and what part may be due to changes in other external factors. Therefore, according to the NCHRP Report 500 (16) definition, these intersection treatments are still considered to be either “tried” or “experimental” and 150

should be properly evaluated in order to move them into the “proven” category. In some cases, it was hard to believe that more implemen- tation of the innovative designs has not occurred and that evaluations of their effects could not be found. For example, offset left-turn lane design guidance is included in the Green Book, the MUTCD, and a number of STA design manuals. With guidance this widely available, we expected to find a number of examples of implementation and evaluations of their safety effects. Unfortunately, it seems relatively few STAs have used offset left-turn lanes at TWSC rural express- way intersections and, of those that have, only one was able to provide data examining the safety effects. It is believed that experimentation with innovative safety strategies is being hampered by the lack of substantial proof that the turn lanes improve safety without creating other opera- tional issues. For these innovative intersection designs, the development of a data-collection protocol is recommended. A statistically sufficient sample should be consistently con- structed, tested, and monitored for a sufficient time period to perform valid safety evaluations. Priority should be given to strategies that address right-angle collisions and their cause (gap selection). With solid evidence of safety im- provement associated with these design alternatives, more STAs may be willing to support their implementation. It is recommended that a pooled-fund study be organized and managed through TRB that would start to deploy and rig- orously evaluate some or all of the innovative rural ex- pressway intersection treatments discussed in this report. In addition to determining their safety effects, more re- search is also necessary to determine the conditions under which each treatment should be considered and under which each would be expected to fail in terms of safety or operations. A focus group was held in December 2006 following a multi-state video conference in which the initial results of the case studies were shared. At the conclusion, a vote was taken relative to which of the countermeasures STAs would like to see more information and research. In addition to the 10 treatments discussed in the case studies, 3 countermeasures were added to the ballot as a result of discussions during the focus group: expressway roundabouts, continuous flow T-intersections, and continuous flow four-legged intersections. Representatives from 13 STAs (Alabama, Iowa, Maryland, Michigan, Minnesota, Mississippi, Missouri, New York, North Carolina, Ohio, Texas, Virginia, and Washington) plus FHWA (Turner-Fairbank) participated, with each agency casting 10 votes, but no more than 4 votes from one agency could be placed on a single countermeasure. The maximum number of votes a countermeasure could receive was 56. The results of the voting are given in Table 54. J-turn intersections ranked first overall, receiving 19% of the votes. Therefore, it is recom- mended that J-turn intersections be the first countermeasure to undergo rigorous statistical evaluation. 151 RANK TREATMENT VOTESRECEIVED PERCENT 1 J-Turn Intersections 27 19.29 2 Freeway-Style Advance Intersection Guide Signs 20 14.29 3 Offset Left-Turn Lanes 15 10.71 4 Left-Turn Median Acceleration Lanes (MALs) 14 10.00 5 Offset T-Intersections 10 7.14 5 Static Roadside Markers 10 7.14 7 *Expressway Roundabout 9 6.43 8 Dynamic Advance Intersection Warning Systems 8 5.71 8 Intersection Decision Support (IDS) Technology 8 5.71 10 Offset Right-Turn Lanes 7 5.00 11 Jughandle Intersections 6 4.29 12 *Continuous Flow T-Intersections 3 2.14 12 *Continuous Flow Four-Legged Intersections 3 2.14 TOTAL VOTES = 140 100 *Suggested treatments during focus group Table 54. Research prioritization results (December 2006).