Design Guidance for Intersection Auxiliary Lanes (2014)

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101 Conclusions, Recommendations, and Suggested Research Project Summary Researchers reviewed recent literature, state design manuals, and multiple editions of the AASHTO Green Book to determine the state of the practice and the basis for it. The research team also interviewed practitioners at state departments of transpor- tation to gain additional insight into current design practices for deceleration lanes, multiple turn lanes, and channelizing island design. Using information from those activities, along with input from the project advisory panel, the research team conducted field studies on double left-turn lane operations and deceleration lane operations. Based on the findings from those field studies in conjunction with current practice and recent lit- erature, the research team made recommendations for revisions to Chapter 9 of the 2011 AASHTO Green Book. Literature Review Researchers consulted various sources of literature, review- ing completed research findings (both recent and distant) and current research projects to determine the current state of the art and state of the practice on the design of auxiliary lanes. Results of the literature review, found in Chapter 2 of this document, focused on a selection of topics: • Warrants (i.e., installation guidelines) for left-turn and right-turn lanes. • Design of auxiliary lane components (i.e., deceleration, storage, and taper). • Safety. • Offset turn lanes. • Intersection sight distance. • Channelization. • Effect of skew. • Multiple turn lanes. • Bypass lanes. • Passing lanes. • Alternative intersection designs. • Acceleration lanes. • Design tools. State of the Practice—Review of Online Design Manuals As part of Task 2 efforts, the research team conducted a state-of-the-practice review of current design consider- ations. A review of state online design manuals was con- ducted to identify (1) what is being discussed at the state level and (2) current design criteria being used for inter- section auxiliary lanes. Table 2-10 summarizes the guidance provided in the state manuals. State of the Practice—Discussions with Practitioners To identify those practices and evaluations not adequately documented in traditional literature and/or design manuals, the research team contacted representatives from a selection of state departments of transportation to inquire about cur- rent practices and potential guidance needs, based on their professional experience and the policies of their respective departments. The information presented in Chapter 3 reflects the participants’ responses as provided to the research team. This state-of-the-practice questionnaire provided some insights into current issues related to the design of auxiliary lanes. The topics covered in the questionnaire were based on feedback from the project’s advisory panel, and responses were received from 11 of the 12 state DOTs contacted. Fol- lowing are findings from the responses received. Deceleration Length • There are multiple ways to define a deceleration lane. When discussing deceleration lane design, care should C H A P T E R 7

102 be taken to ensure that all participants are using the same definition. • Reasons for installing a deceleration lane and/or determin- ing its length are typically based on various other geometric and traffic considerations, capacity, and speed being among the most common. • Agencies use numerous sources as the basis for their decel- eration lane guidelines. The AASHTO Green Book and NCHRP Report 279 were the most common and have simi- lar roots in Harmelink’s work, but other sources are also used, including state-specific manuals and other national documents. • When the preferred dimensions of a deceleration lane can- not be accommodated within a particular site, the decision for how to make adjustments and reductions is typically a qualitative one, though the factors that contribute to that decision are not always well defined. • Respondents were generally satisfied with existing guidance, though there were suggestions to include better methods of projecting future turning volumes for determining storage and better guidance on the type of widening or shadowing that is appropriate for a given auxiliary lane. Multiple Turn Lanes • Double Left-Turn Lanes: – Most agencies have some kind of guidance regarding double left-turn lanes, though it may not be very detailed. – The decision to install such a treatment is frequently based on the current or expected turning demand, but signalization is also a typical criterion. – Guidance on the design of receiving lanes is often described as the width of the turning curve, though it may also be described in terms of receiving lane width. – The capacity of a double left-turn lane was generally viewed to be less than twice that of a single lane, but the exact value of that capacity was not universally agreed upon. – Guidance on how to adjust the three components of a double left-turn lane was widely varied, if available. The adjustments for storage, deceleration, and taper of a dou- ble left-turn lane were often determined qualitatively or on a case-by-case basis. – Some of those issues were reflected in respondents’ sug- gestions for added guidance, desiring information on lengths for given design speeds or for urban vs. rural settings. • Triple Left-Turn Lanes: – Existing guidance on triple left-turn lanes is very limited, and in some states the treatment is heavily discouraged or prohibited. – The guidance that does exist is typically similar to that found for double left-turn lanes, with additional con- siderations for the turning curve and receiving lanes. • Double Right-Turn Lanes: – Existing guidance for double right-turn lanes is less common than that for double left-turn lanes but more common than triple left-turn lanes. – Design guidance for a double right-turn lane is fre- quently similar, if not identical, to guidelines for design- ing double left-turn lanes, though site-by-site analysis is also important. – Several respondents desired additional information, such as consideration of pedestrians, appropriate lengths for given design speeds, and warrants for installation. Island Design for Right-Turn Lanes • Guidance on island design is found in many states, though that guidance is not always very detailed. • Some states’ guidelines contained recommendations for island sizes, approach angles, turning volumes, and pedes- trian provisions, but many materials were more general in nature. • Preferences for flat-angle entry to the cross street were slightly higher than preferences for entry closer to a right angle. • Various traffic and geometric characteristics contribute to the design of a turning island, including traffic volume, intersection size and angle, pedestrian accommodation, sight distance, and speed. • Little additional information was suggested for inclusion in guidance documents that is not currently provided, although there was a concern for pedestrian accommodation when an island is installed. • Respondents suggested that some island designs could be improved with further consideration of large vehicles, accommodating pedestrians and older road users, approach angle and taper, and sight distance. Typical Designs As part of the questionnaire sent to key state transporta- tion agencies, respondents were asked to identify locations with installations that would be considered best-practice sites. These best-practice sites were to demonstrate preferred design treat- ments for five design categories: island design, deceleration lane design, double left-turn lane design, triple left-turn lane design, and double right-turn lane design. Colorado, Florida, Maine, Minnesota, North Carolina, and Washington all iden- tified locations for consideration. An underlying question associated with the identifica- tion of these locations centered on whether the Green Book provides sufficient guidance for implementing the treatment.

103 Each of the identified sites was examined using aerial imag- ery to gain a better understanding of the key features of the design, as implemented. A single site considered representa- tive of the design treatment was identified for each of the five design categories. This representative site was examined in detail through a case study approach. The case studies focused on the key design features associ- ated with the treatment under consideration. To the extent pos- sible, these design features were quantified and referenced to the appropriate standards. A review of each of the completed case studies was then done to determine if the guidance provided by the Green Book would have been sufficient or if supplemental guidance would have been necessary to complete the design. In those cases where supplemental guidance was necessary, this supplementary guidance has been summarized. The evaluation of best-practice sites that demonstrate preferred design treatments for five design categories—island design, deceleration lane design, double left-turn lane design, triple left-turn lane design, and double right-turn lane design— show that the guidance found in the 2011 Green Book is ade- quate for island design. However, supplemental guidance may be needed to address adequately the design needs of pro- viding deceleration lanes in a flared intersection configura- tion, which occurs when adding a turn lane for an undivided roadway, and to address clearance distances for multiple turn lanes. Preliminary Review of Issues As part of the initial tasks for this project, the research team identified a list of issues to consider for further study. This list was provided to the panel and discussed during a conference call. At the conclusion of the conference call, the following issues were prioritized for potential Phase II studies: • Multiple turn lanes. • Deceleration lane (validation of components, safety/ operations of different lengths). • Island design/channelization for right-turn lanes. • Pedestrian/bicycle/complete street issues. Field Studies and Final Green Book Review In Task 3 of the project, the research team developed an interim report that described the activities and findings from Tasks 1 and 2. That interim report was accompanied by a list of possible research topics for review by the project advisory panel; in accordance with previous direction from the panel, possible research topics were focused on aspects of multiple left-turn lanes and left-turn deceleration lanes. The research team and the panel met to discuss the findings from Tasks 1 and 2 to determine the emphases of the field studies in Phase II of the project. The panel selected two field studies for completion in Phase II: • Operational study on double left-turn lanes. • Operational study on deceleration lanes. Both studies were chosen with the intent that they would produce recommendations for revisions to the Green Book, and those recommendations are included with other sug- gested changes identified in the final task of the project, the Green Book review. Conclusions Double Left-Turn Lane Operations The goal of the double left-turn lane operational study was to determine the effects of geometric characteristics on double left-turn lane operations, as measured with saturation flow rate, lane distribution, and driver behaviors. Receiving leg width, left-turn lane width, and downstream friction type and distance were the key geometric variables studied. Identifying sites with the desired range of receiving leg width was the most difficult of the study variables to satisfy, although finding sites with an aver- age double left-turn lane width greater than 12 ft also proved challenging. Data from 26 sites in three states (Arizona, Cali- fornia, and Texas) were used in the analyses. The data collection method was video recording. The ITE Manual of Transportation Engineering Studies (79) procedure guided the determination of the saturation flow rate. The procedure requires that each cycle must have more than seven vehicles and only passenger cars in the traffic stream are to be considered in the determination of saturation flow rate. Because of challenges with obtaining sufficient sample size, queues five to 10 vehicles in length were included in the analysis. The number of vehicles in the queue was a variable added to the model in case the shorter queues had a different saturation flow rate than the lon- ger queues. The time each left-turning vehicle crossed the stop bar was logged, and these times were used to determine the headway between following vehicles. Also recorded was whether the vehicle was a truck as well as whether the vehicle was not in the queue at the start of the cycle. If either case was true, then the queue was eliminated from the study. The times between vehicles were used to calculate the satu- ration flow rate. Saturation flow rate represented the number of vehicles served by one lane over 1 hr of green time. It was calculated using the headway between following vehicles when all vehicles being considered were passenger cars and were present at the start of the green phase. The headways for the first four vehicles were dropped from the calculation. A total of 10,023 saturation flow rate values were available for study. The average double

104 left-turn lane saturation flow rate for these 10,023 data points was 1775 pcphgpl. At some DLTL intersections, drivers may choose one left- turn lane over another left-turn lane in anticipation of a turn at a downstream intersection or because of familiarity with a downstream friction point. The data available in this study were used to investigate if geometric elements are associated with how drivers distribute within the double left-turn lanes. A variable called lane share was created to calculate the percent- age of the volume present within a cycle to each lane. If the queues were equal between the lanes, the lane share variable for Lane 1 was 50% and 50% for Lane 2. This lane share variable was aimed at determining the proportion of vehicles that use a lane out of all left-turn vehicles recorded during a cycle in order to provide a good measure of the distribution of left-turn demand of passenger cars across the double left-turn lanes. Driver behavior may be related to the geometric design characteristics of the double left-turn lanes, which could affect the saturation flow rate (i.e., operations) or the safety of the intersection. For example, slow-to-start behaviors may decrease the saturation flow rate and increase the potential for rear-end crashes. Driver behaviors of interest for double left- turn lanes were identified from the literature in conjunction with the engineering judgment of the research team. Techni- cians then watched the video and documented whenever one of those driver behaviors occurred. Key findings from the analyses of operations at double left-turn lanes include the following: • The lane variable was found to be not significant, which means that the inside and outside lane saturation flow rates were similar. • The number of vehicles in the queue was also not significant. In other words, whether the queue length was five vehicles or ten vehicles, similar saturation flow rates were measured after controlling for variations caused by other variables within the model. • Because U-turns require drivers to slow more than they would for a left turn, it is reasonable to assume that a U-turn will also take more time and therefore negatively affect satu- ration flow rate. The model found that for each additional U-turning vehicle within the left-turn queue, saturation flow rate decreased by 56 pcphgpl. • The analysis of the effects of the friction point type and location revealed that the analysis needed to include a new variable. The new variable accounted for a dedicated lane added at the end of a channelized right turn. While the turning vehicles were constrained to two lanes at the start of the receiving leg, a review of the video data revealed that drivers in the outside lane would angle their vehicle to make a smooth entry into the new lane. This behavior resulted in higher saturation flow rates, as demonstrated with the variable being significant. The model results indicated that the addition of this new lane resulted in an increase in saturation flow rate of about 50 pcphgpl. • The Highway Capacity Manual (8) indicates that wider lane widths are associated with higher saturation flow rate. One of the findings from this DLTL study was that the width of the left-turn lanes did not significantly affect the saturation flow rate. This finding could be construed to mean that narrow lanes can be used without affecting operations. In making this interpretation, however, a key component of the study design is not represented. The recommended method to determine saturation flow rate requires the elimination of a queue if a heavy vehicle is present within the queue. Therefore, within this study, while the operations of queues with only passenger cars were similar for the various left- turn lane widths studied (9.5 to 13 ft), the operations of queues that include heavy vehicles (trucks or buses) may have different results. • The width of the receiving leg represents the visual target for the left-turning drivers. For the sites included in this study, the width of the receiving leg ranged from 24 ft to 54 ft. The analysis did find that the width of the receiv- ing leg affected the saturation flow rate. While significant, the incremental difference in saturation flow rate for an incremental increase in leg width was small. The pattern of increasing saturation flow rate for increasing receiving leg width was examined to try to identify if there were dimen- sions where a sizable increase in saturation flow rate occurs. The change point detection analysis based on predicted satu- ration flow rates identified a receiving leg width of 36 ft as the change point. When the receiving leg width was between 24 and 36 ft, the average saturation flow rate was 1725 pcphgpl, while a receiving leg width of 40 to 54 ft was associated with an average saturation flow rate of 1833 pcphgpl. Determin- ing whether the benefits of the additional saturation flow outweigh the costs (e.g., maintenance, construction, and/or right-of-way) was not a component in this study; therefore, the benefit-cost ratio may be very limited. • As demand increases, the selection of which left-turn lane to enter may be more of a reflection of drivers selecting the lane with the shorter queue rather than being concerned with downstream conditions. Therefore, the evaluation on lane distribution only used the queues in which fewer than 11 vehicles were present within the queue. Several variables were found to be significant; however, almost all of the prediction was accomplished by the Lane 1 and the Lane 2 volumes, indicating that drivers are making lane selection based on the length of queues present when the driver approaches the intersection. • For the 18 sites included in the driver behavior review, the most common behavior was lane changes before the friction location, followed by lane changes at the friction location. As

105 expected, many sites with driveways/intersections as the friction point were associated with high numbers of lanes changes. The behaviors that reflected conflicts or potential conflicts were reviewed to identify if there were common elements. The three sites with the largest number of these types of behaviors all had a friction point within 150 ft of the intersection. Deceleration Lane Operations The goal of the deceleration lane study was to determine the effects of taper length and posted speed limit on approach speeds and deceleration rates of left-turning vehicles, as com- pared to those described in the Green Book. Deceleration length, through lane width, left-turn lane width, and operating speeds at key points along the approach through the left-turn lane were also measured and included in the analysis. Identify- ing sites with medians that contained a distribution of posted speed limits and taper lengths was the most challenging part of site selection, though the research team also sought to identify sites where proximity improved efficiency of the data collec- tion effort. Data from 12 sites in four states (Alabama, Florida, Mississippi, and Texas) were used in the analyses. The primary data collection method was video recording. Researchers observed and recorded images of vehicles that completed left turns at signalized intersections with medians. The recordings were used to document the times at which each vehicle reached the beginning of the left-turn taper, the end of the taper, a predetermined point within the deceleration lane, and the stop line. Using the elapsed time to travel between those locations and the corresponding distances, researchers calculated speeds and deceleration rates of the left-turning vehicles. To verify these calculations, researchers also collected limited amounts of data with lidar and with GPS, comparing the data from those efforts to that of the video. Researchers also used an automated traffic counter to collect spot-speed data upstream of the left-turn lane to provide an indication of the approach speeds of left-turning vehicles as well as likely speeds of through vehicles as they continued through the intersection. Following data reduction and quality control checks, data from 410 left-turning vehicles were analyzed to investigate three key deceleration lane design guidelines in the Green Book: • The speed differential between turning vehicles and follow- ing through vehicles is 10 mph when the turning vehicle clears the through-traffic lane (see Note 3 in Table 6-2). • The values for deceleration length are based on a 5.8 ft/s2 average deceleration while moving from the through lane into the left-turn lane (see Note 4 in Table 6-2). • The values for deceleration length are based on a 6.5 ft/s2 average deceleration after completing the lateral shift into the left-turn lane (see Note 4 in Table 6-2). Key findings from the analyses of operations at decelera- tion lanes were: • The Green Book currently states that a 10-mph differential is acceptable on arterials, but drivers commonly had speed differentials greater than 10 mph in this study. Furthermore, the higher speed differentials in this study occurred for vehi- cles traveling at higher speeds. The Green Book describes the 10-mph differential in relation to a comfortable deceleration for the driver, while support for using 10 mph as the thresh- old for speed differential may be better stated in terms of reducing the likelihood of a crash. • The speed differential was significantly and positively related to the upstream speed. Other variables, such as posted speed limit, deceleration length, and taper length also had various effects on speed differential in the statistical models, and the general effect of deceleration length appears to be consistent with the Green Book text that describes higher differentials with shorter length, but those effects were not statistically sig- nificant at the 0.05 level. • The Green Book deceleration rate of 5.8 ft/s2 prior to the end of the taper was within the range of average rates at the study sites, but eight of the 12 sites had an average rate higher than 5.8 ft/s2. The 50th percentile rate was approximately 6.1 ft/s2 for low-speed sites and 6.7 ft/s2 for high-speed sites. In addition, 85% of observed drivers at high-speed sites decelerated at a rate of 4.2 ft/s2 or greater up to the end of the taper. • The deceleration length (at low-speed sites), the speed at the upstream counter, and the speed in the taper were all significant in affecting the rate of deceleration prior to the end of the taper. Coefficients for decelera- tion length and speed in the taper have negative signs because as these variables are reduced, one expects to see an increase in deceleration rate. Conversely, deceleration through the end of the taper increases as the upstream speed increases. • The recommended taper rates and lengths in the Green Book are presented in conjunction with the speeds of the vehicles using the left-turn lane, so the design of the taper area leading into the full-width deceleration lane recog- nizes an influence of speeds, but the design guidelines in the text are not directly tied to the 5.8 ft/s2 value presented in Table 9-22. Many of the study sites provided a taper length consistent with the recommended taper rates, but data from this study show that drivers commonly traveled through the taper at deceleration rates higher than 5.8 ft/s2 for taper rates between 6:1 and 18:1. • A design that accommodates decelerating at 4.2 ft/s2 dur- ing the lateral movement into the turning lane provides for a more gradual, controlled deceleration, but a higher deceleration rate (closer to 6.5 ft/s2 for half of the observed

106 drivers or 10 ft/s2 for the most aggressive drivers) could be acceptable if site constraints or other factors dictate a shorter length. The tradeoff for the shorter length, however, would occur in one or both of the following forms: – Less aggressive drivers would begin their deceleration earlier, either through coasting or applying the brake further upstream of the beginning of the taper, increas- ing the speed differential between turning and through vehicles. – Some drivers would accomplish more of their decel- eration after lateral movement, leading to much higher deceleration rates approaching the stop line and/or the back of the queue. • Deceleration length and the vehicle speed in the taper were found to be statistically significant in determining decel- eration rate within the full-width deceleration lane. Decel- eration length was negatively related to deceleration rate (i.e., shorter lengths had higher deceleration), while speed was positively related; both of those relationships are as expected. • Compared to the 6.5 ft/s2 rate noted in the Green Book, approximately two-thirds of the drivers observed making left turns at the study sites decelerated at greater rates to come to a stop at the stop line. Data from this study indi- cate that a designer could produce a left-turn lane design that is associated with a deceleration rate of 6.5 ft/s2 and it would accommodate the current behavior of 85% of left- turning drivers at high-speed sites and half of drivers at low-speed sites. • In the Gates et al. study (95), deceleration was measured for drivers going straight. In this study, deceleration was measured for left-turning drivers while changing lanes in preparation for a stop at the stop line. This study found slightly slower deceleration rates as compared to the Gates et al. study. The deceleration rates for vehicles going straight on roads above 40 mph were 9.2 ft/s2 (15th percentile), 10.9 ft/s2 (50th percentile), and 13.6 ft/s2 (85th percentile). The deceleration rates for left-turning passenger cars that changed lanes on roads with posted speed limit greater than 45 mph were 6.5 ft/s2 (15th percentile), 8.9 ft/s2 (50th percen- tile), and 11.4 ft/s2 (85th percentile). Final Green Book Review A complete description of all recommended improvements to Chapter 9 of the 2011 Green Book is provided in Appendix A of this report. This section summarizes the changes identified by the research team as a result of the activities on this project. Section 9.3, Introduction (pages 9-8 to 9-10) • Recommended minor changes in text, including replacing the term “free-flow right-turn lanes” with “channelized right-turn lanes” to correspond with results from NCHRP Project 03-89 (28). Section 9.3.1, Three-Leg Intersections (pages 9-10 to 9-14) • Recommended expanded discussion of bypass lanes, includ- ing a cross-reference to warrants suggested for inclusion in Section 9.7.3, based on research in NCHRP Report 745 (9). • Recommended revisions to some existing diagrams to improve legibility, provide additional detail, and add conflict diagrams. Section 9.3.2, Four-Leg Intersections (pages 9-14 to 9-19) • Recommended additional cross-references to guidance on auxiliary lane design in Section 9.7 for improved clarity. • Recommended updating the text to better reflect current practice and guidance (including the Highway Design Hand- book for Older Drivers and Pedestrians) on skew angle, which should not be less than 75 degrees. • Recommended revisions to some existing diagrams to improve legibility, provide additional detail, and add conflict diagrams. Section 9.4.2, Alignment (page 9-27) • Recommended updating the text to better reflect current practice and guidance (including the Highway Design Hand- book for Older Drivers and Pedestrians) on skew angle, which should not be less than 75 degrees. Section 9.4.3, Profile (page 9-27) • Recommended updating the text to include specific refer- ences to federal accessibility guidelines. Section 9.6.1, Types of Turning Roadways (pages 9-55 to 9-88) • Recommended inserting a new subsection on page 9-55 on channelized right-turn lanes, to improve the level of detail currently found in the Green Book, based on research in NCHRP Project 3-89 (28). • Recommended a revision to the text on page 9-80 to include environmental issues as a factor in justifying tradeoffs in design, as recommended by NCHRP Synthesis 422 (101). • Recommended a change in the description of curb radii and refuge islands that accompanies Green Book Figures 9-34 and 9-35 on page 9-88. References are provided to guidelines from AASHTO, ITE, and Virginia DOT.

107 Section 9.6.2, Channelization (pages 9-93 to 9-94) • Recommended addition to the text on page 9-93 to promote the use of median storage for vehicles turning left from a minor road onto a divided highway. • Recommended updating the text to better reflect current practice and guidance (including the Highway Design Hand- book for Older Drivers and Pedestrians) on skew angle, which should not be less than 75 degrees. • Recommended the use of “storage” instead of “refuge” to describe the storage distance for turning vehicles, to prevent confusion with refuge areas for pedestrians or bicyclists. Section 9.6.3, Islands (pages 9-97 to 9-104) • Recommended a revision of Green Book Figure 9-37 to improve legibility and correct perceived alignment errors. • Recommended adding text to describe the appropriate length of the transition taper as the value calculated by Equation 3-37 or 3-38 on page 3-134. Most state design manuals use a form of those taper equations, and the addi- tion would provide clarification as well as consistency with current practice. Section 9.6.4, Free-Flow Turning Roadways at Intersections (page 9-106) • Recommended removal of this section, including Figure 9-42. This information deals more with the exit design from either a freeway or grade-separated arterial, which is already covered in Chapter 10 (Section 10.9.6 Ramps). Further, it confuses the issue with relation to corner islands. Section 9.7.1, General Design Considerations (pages 9-124 to 9-125) • Recommended revision to the text to clarify the description of appropriate use of acceleration lanes. Section 9.7.2, Deceleration Lanes (pages 9-125 to 9-130) • Recommended revision of the section on deceleration length on pages 9-125 to 9-127 to incorporate the findings from this project’s Task 4 deceleration field study. This included adding a subsection on perception-reaction distance. It also included replacing Figure 9-48 and Table 9-22 for clarity and consistency. • Recommended revision of the section on taper length on pages 9-127 to 9-130 to better define the purpose, dimen- sions, and design of a taper used to add a left-turn lane. This revision also incorporates the findings from this project’s Task 4 deceleration field study. Section 9.7.3, Design Treatments for Left-Turn Maneuvers (pages 9-131 to 9-139) • Recommended changing the structure of the section beginning on page 9-131 to better reflect its content. Much of the subsection entitled Guidelines for Design of Left-Turn Lanes really focuses on installation of left-turn lanes. This subsection could be split into multiple parts, to include discussion of installation, warrants, and general design principles. The existing text in the first two para- graphs is on installation. The suggested revised text is pre- sented as a subsection on warrants for left-turn lanes and bypass lanes, based on research in NCHRP Report 745 (9). The remaining text can then be the Guidelines for Design of Left-Turn Lanes subsection. • Recommended revising the text in the section on page 9-139 to better describe the capacity benefits and design consider- ations for multiple left-turn lanes, based on findings from the double left-turn lane operational study in Task 4 of this project. Section 9.8.1, General Design Considerations (page 9-140) • Recommended revising this section to include discussion on the value and benefits of directional and bidirectional crossovers, modification of median openings, and vehicle storage in wide medians. There is currently no guidance listed for under which conditions a median opening should be eliminated or when it should be made directional. This is compounded by the fact that there is no discussion of the difference between a bidirectional (conventional) crossover and a directional crossover. Additional sources for informa- tion on median openings are in the following: Michigan DOT Geometric Design Guide 670 (102), Michigan DOT Road Design Manual (103), FHWA Alternative Intersec- tions/Interchanges: Informational Report (104), and NCHRP Report 650 (105). There is no discussion of the impact of providing markings or a divisional island for vehicle storage in the median. Guidance regarding the impact of proper pavement markings on driver behavior should be added as available in NCHRP Report 650 (105). Section 9.8.2, Control Radii for Minimum Turning Paths (page 9-144) • Recommended adding text to emphasize the safety impacts of minimizing the median opening length. Much of the discussion focuses on curve radii for the median design but forgets that the overall length of the opening is important as well. NCHRP Report 650 provides discussion regarding median design (105).

108 Section 9.8.3, Minimum Length of Median Opening (page 9-149) • Recommended revising text for clarity and to provide a reference to supporting research. Section 9.9.1, General Design Considerations (pages 9-155 to 9-157) • No changes were specifically recommended, but research- ers recommended including a placeholder for a guidebook scheduled to be completed in an ongoing research proj- ect. The placeholder was added as a reminder to review this resource when it becomes available and determine if it should be added as a reference resource to this section. Section 9.9.2, Intersections with Jughandle or Loop Roadways (pages 9-157 to 9-160) • Recommended additional text and a table to provide guid- ance on the spacing of jughandle intersections and the corresponding primary intersection. • Recommended revising Figures 9-60 and 9-62 for clarity and to provide a conflict diagram. • Recommended text revisions to remove inconsistencies in terminology and to better describe operations at the minor intersections. • Recommended adding a reference to the Alternative Intersections/Interchanges: Informational Report (104). Section 9.9.3, Displaced Left-Turn Intersections (pages 9-160 to 9-161) • Recommended additional text and a table to compare conflict points for continuous-flow intersections and tra- ditional intersections. • Recommended adding a reference to the Alternative Intersections/Interchanges: Informational Report (104). Section 9.9.4, Wide Medians with U-Turn Crossover Roadways (pages 9-162 to 9-164) • Recommended additional text and a replacement for Fig- ure 9-65 to better discuss the restricted crossing U-turn intersection. • Recommended adding a comment about using loons to accommodate larger vehicle U-turns with narrow medians. • Recommended adding text, tables, and figures to better describe the median U-turn intersection and the restricted crossing U-turn intersection. • Recommended adding a reference to the Alternative Intersections/Interchanges: Informational Report (104). Section 9.9.5, Location and Design of U-Turn Median Openings (pages 9-165 to 9-166) • Recommended revisions to the text and Table 9-30 to correct inconsistencies in the values for median widths. • Recommended updating the text on wide medians and adding a new figure to show a typical loon design in order to better represent current practice. • Recommended revising text to describe the location of midblock median U-turn openings to improve consistency with current practice. • Recommended revising text and adding figures to bring description of multiple U-turn lanes into agreement with the state of the practice. Section 9.10, Introduction (pages 9-167 to 9-169) • Recommended adding a paragraph to the end of the section to emphasize the benefits of public outreach and education. Section 9.10.2, Fundamental Principles (pages 9-173 to 9-175) • Recommended adding a paragraph on right-turn bypass lanes to the end of the section entitled “Lane Balance and Continuity” to fill an information gap. • Recommended adding a paragraph on turbo roundabouts to the end of the section entitled “Appropriate Natural Path” to fill an information gap. • Recommended adding a paragraph on large, oversized, and superload vehicles to the end of the section entitled “Design Vehicle” to fill an information gap. • Recommended adding a reference to NCHRP Report 674 (65). Recommendations The following sections present the recommendations from the field studies conducted in this project. Double Left-Turn Lane Operations Researchers used the findings from this study to develop rec- ommendations on geometric design features that affect double left-turn lane performance. Potential recommendations include the following: • The Green Book states that the capacity of double left-turn lanes is approximately 180% of that of a single median lane. Per the Highway Capacity Manual (8), the base satu- ration flow rate for a metropolitan area with population of 250,000 is 1900 pcphgpl and the left-turn adjustment

109 factor is 1/1.05. Comparing the single-lane saturation flow rate (1900/1.05 = 1810 pcphgpl) to the average saturation flow rates for the double left-turn lanes sites in this study (1774 + 1776 = 3550 pcphgpl) results in a value (3550/1810 = 1.96 or 196 percent) that is greater than 180 percent. • The Green Book states that the receiving leg of the inter- section should have adequate width to accommodate two lanes of turning traffic and that a width of 30 ft is used by several highway agencies. Early literature by Neuman (5) stated the throat width for the turning traffic is the most important design element, and that because of the offtracking characteristics of vehicles, a 36-ft throat width is desirable for acceptance of two lanes of turning traffic. In constrained situations, 30-ft throat widths are accept- able minimums. Within this study, the pattern of increas- ing saturation flow rate for increasing receiving leg width was examined to try to identify if there were dimensions where a sizable increase in saturation flow rate occurs. The method concluded that the change point occurs between receiving leg widths of 36 ft and 40 ft. • Additional discussions or cautions in the section on multiple turn lanes: – Double left-turn vehicles, turning into a receiving leg of two lanes where a third lane was being added as a dedi- cated downstream lane from a channelized right-turn lane, were observed to move into the additional lane as soon as physically possible, even across a solid white line. – The number of U-turning vehicles has a significant impact on the operations of double left-turn lanes. Deceleration Lane Operations Researchers used the findings from this study to develop recommendations on geometric design features that affect deceleration lane performance. Potential recommendations include the following: • The Green Book states that a 10-mph differential between turning and through vehicles is commonly considered accept- able on arterial roadways, though higher speed differentials may be acceptable on collector highways and streets due to higher levels of driver tolerance for vehicles leaving or enter- ing the roadway due to slow speeds or high volumes. Given that higher speed differentials also occur on high-speed arte- rials as found in this research, it may be useful for the Green Book to clarify on what basis that differential is “acceptable” and add the references to the research or other guidance documents that support the explanation. For example, refer- encing research by Soloman (98) or others that describe the increased likelihood of crashes as speed differential increases above 10 mph establishes a basis for recommending that designers provide left-turn lane designs that do not require speed differentials greater than 10 mph. This research also identified smaller speed differences for sites with longer com- bined taper and deceleration lengths. • The support for selecting the deceleration values of 5.8 ft/s2 and 6.5 ft/s2 noted in Table 9-22 of the Green Book is not directly referenced in the table or described in the guidelines in the accompanying text. Drivers are able to comfortably decelerate at greater rates, though a design that accommodates lower rates provides a more conserva- tive design that is less demanding on drivers and contains more provision for storage of queues. However, if that con- servative design is the desired result, the guidelines in the Green Book should contain additional text and references to relevant research supporting the use of those values. Based on the results of this study, a two-stage deceleration process that uses rates of 4.2 ft/s2 during lateral movement into the turning lane and 6.5 ft/s2 within the deceleration lane would accommodate most drivers. In constrained locations, a single deceleration rate of 6.5 ft/s2 over the full deceleration length would suffice. Providing designs that require greater deceleration rates would likely result in larger speed differences between turning and through vehicles as turning drivers choose to complete more of their deceleration upstream of the taper. Review of Green Book Researchers completed two reviews of Chapter 9 (Inter- sections) of the 2011 AASHTO Green Book. Comparing the existing guidance in Chapter 9 with results from this research project as well as recent research and current practice, the research team compiled a list of recommended revisions to text, figures, and tables. Specifically related to the findings from this research project, researchers recommended that the Green Book guidance be revised to reflect new recommendations for: • Typical and constrained deceleration lengths at left-turn lanes. • Deceleration rates associated with recommended decelera- tion lengths. • Text to better describe the capacity benefits and design considerations for multiple left-turn lanes. Suggested Research Based on the findings from the literature review, state-of-the- practice review, and field studies on this project, researchers sug- gest the following topics to be considered for further research: • The safety performance of double left-turn lanes (i.e., where crashes occur and how they can be prevented).

110 This project studied operational characteristics and their relationship to design elements. A similar study on what design characteristics (e.g., lane width on approach, receiving lane/leg width, separation between opposing left-turn lanes, presence of pavement markings, loca- tion of downstream friction points) affect the type (e.g., sideswipe, rear-end) and frequency of crashes could be equally beneficial. • The characteristics of lane-change maneuvers into the left- turn lane (i.e., where maneuvers occur relative to the begin- ning and end of the taper). Results from the deceleration study in this project support other findings and guidance that indicate the lane-change maneuver is not strictly con- fined to the taper area. To provide the most useful guidance on taper length and its relationship to speed differential when a turning vehicle clears the through lane, a study that explores lane-change location with respect to taper length would be very informative. • Use of auxiliary/acceleration lanes for vehicles making a left- turn from a minor road. Some research has been conducted on this topic, but the conditions under which auxiliary/ acceleration lanes provide the greatest safety and opera- tional effects have not been fully explored. Guidance on when to provide acceleration lanes and how to determine the appropriate design dimensions has potential for wide- spread use in the design of rural high-speed intersections with two-way stop control. • Design of channelization elements. The literature review and state practitioner interviews indicated that there is a need for more detailed guidance on the design of chan- nelized right-turn islands and median openings. These designs have benefits for capacity and access, but they also have the potential to affect pedestrian crashes (at right- turn islands) and turning crashes (at median openings). A better understanding of what design dimensions provide the best operational and safety benefits will help design- ers know what elements are best suited for conditions at specific locations. • Design differences between right-turn and left-turn lanes. It is commonly assumed that left-turn lanes and right- turn lanes are similar enough that most design elements can generally be designed the same way, but differences between the two types of lanes are not formally docu- mented and it is not clear which differences are important enough to specifically include in the Green Book. • Benefits of offset left-turn lanes on safety and other per- formance measures. Previous research has identified some operational benefits to offset left-turn lanes, along with sug- gestions for selected design elements. However, it is unclear what effect offset left-turn lanes have on safety, and whether there are other benefits (e.g., improvement in perceived sight distance, reduced driver workload). • Appropriate distance between simultaneous left turns, espe- cially for multiple turn lanes. Selected guidelines discuss the distance between opposing left-turn lanes, but a comprehen- sive study was not found in the review during this project. That distance is a particularly noteworthy design element in the design of intersections with simultaneous turns from double left-turn lanes. • Auxiliary lanes in conjunction with passing lanes on two- lane highways. Prior studies have recommended avoiding locating high-traffic intersections and driveways within the boundaries of a passing lane, but no specific study on the use of auxiliary lanes within passing lane sections was found. Also, no study on tradeoffs between turning lanes and passing lanes was identified. Additional research would be beneficial in answering the question of when to include an auxiliary lane when a passing lane is present, or when to consider a left-turn or right-turn lane instead of a passing lane. • Operational and safety benefits of storage length. As with some other treatments in this list of research topics, the Highway Safety Manual (1) notes that storage length is a treatment with unknown crash effects. The safety benefits of lengthening a left-turn lane are unclear, as are the condi- tions under which the queues from neighboring through lanes affect the design of auxiliary lane lengths at signalized intersections. • Development of auxiliary lane pavement markings for use in multiple turn lane configurations. While this may be viewed as a pavement marking issue, proper location of these markings has the potential to have an impact on the operational characteristics of the multiple turn- ing lanes configuration. Further, the 2009 Manual on Uniform Traffic Control Devices does not provide sufficient guidance for determining the beginning of turn lane lines within the transition area of multiple turn lanes. With- out positive guidance for drivers within the approach transition, proper vehicle alignment is more difficult, causing more turbulent flow, and, thus, may influence operations. • Unsignalized or permissive (e.g., green ball, flashing yellow arrow) phasing of left turns across more than two lanes has been a concern from a safety perspective due to: – Staggering/shadowing of opposing vehicles. – Opposing vehicle speed differentials. – Driver judgment. – Ability to clear opposing through traffic, especially for left turns across three or more lanes at speeds greater than 35 mph. Research could clarify the safety relationships between left turns and the number of opposing lanes for dif- ferent geometric/traffic characteristics such as signalized

111 or unsignalized, presence of left-turn lane, presence of permissive-only or permitted/permissive phasing when sig- nalized, posted speed limit, and others. • Entry angle for right-turn lane. The entry angle can affect how a right-turn lane operates and can affect safety on the approach. A flat-angle entry alignment requires a driver to look left over the shoulder to check for merging traffic and can result in the driver losing sight of a car still in front. The driver may accelerate to accept a gap, not real- izing that the car in front is still there. Additional research and field evaluation should be conducted to determine the relationship between crashes and the angle of the right- turn lane to the cross street. The research should consider cross-street operating speed, cross-street volume, and the age of the driver. • Impacts of trucks on turn lane design. The double left-turn lane study used saturation flow, which removed trucks from the analysis as recommended by the Highway Capac- ity Manual. The characteristics of trucks will obviously have an impact on saturation flow; however, the research need is how these truck characteristics affect safety and operations with respect to other design characteristics, such as lane width or median type. An example question is the following: Are the safety and operations of turn lanes more influenced by the available lane width when a larger percentage of trucks is present?