Design Guidance for Intersection Auxiliary Lanes (2014)

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1 S U M M A R Y Design Guidance for Intersection Auxiliary Lanes Crashes at intersections constitute a large portion of the annual traffic crashes in the United States. Previous experience has shown that auxiliary turn lanes are an effective countermeasure to address some of these crashes, and they are documented as having sizable crash modification factors in the AASHTO Highway Safety Manual. Auxiliary lanes can also be used to increase capacity, improve traffic operations, and increase driver comfort at an intersection. The AASHTO Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) contains some guidance for geometric design of auxiliary lanes at inter­ sections, but more information would benefit the profession. The objective of NCHRP Project 03­102 was to recommend improvements to the guidance provided in the AASHTO Green Book for auxiliary lanes at intersections, thereby leading to improved safety and operations. Specifically, the research team sought to provide additional support for existing criteria and expand the material to fill current information needs. The anticipated result was that the additional guidance would help the profession to realize fully the safety and opera­ tional benefits of auxiliary lanes at intersections and improve the consistency of application for these fundamental road features. 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. To identify practices and evaluations not adequately documented in traditional literature and/or design manuals, the research team interviewed practitioners at state DOTs about current practices and potential guidance needs, based on their professional experience and the policies of their respective departments. The interviews focused on current design practices for decel­ eration lanes, multiple turn lanes, and channelizing island design. Practitioners listed the following among their key responses: • Reasons for installing a deceleration lane and/or determining its length vary based on other geometric and traffic considerations, with capacity and speed being among the most common. • Agencies use numerous sources as the basis for their deceleration lane guidelines, although most sources are based on the AASHTO Green Book and/or NCHRP Report 279. • When the preferred dimensions of a deceleration lane (i.e., taper length, deceleration length, and storage length) cannot be accommodated within a particular site, the decision for how to make adjustments and reductions is typically a qualitative one, although the factors that contribute to that decision are not always well defined. • Most agencies have some kind of guidance regarding double left­turn lanes, although such guidance may not be very detailed. The decision to install such a treatment is frequently based on the current or expected turning demand, but protected­only signalization is also a typical criterion.

2• Guidance on how to adjust the key design dimensions of a double left­turn lane is widely varied, if available. The adjustments for storage, deceleration, and taper of a double left­ turn lane are often determined qualitatively or on a case­by­case basis. As part of the questionnaire sent to key state transportation agencies, respondents were asked to identify locations with installations that would be considered best­practice sites. These best­practice sites were to demonstrate preferred design treatments for five design categories: island design, deceleration lane design, double left­turn lane design, triple left­ turn lane design, and double right­turn lane design. Researchers selected a single site for each of the five design categories and examined it in detail through a case study approach. The case studies indicated that the guidance found in the 2011 Green Book is adequate for island design. However, supplemental guidance may be needed to address (1) the design needs of providing deceleration lanes in a flared intersection configuration and (2) clearance distances for multiple turn lanes. Using the findings from these information­gathering activities, along with input from the project advisory panel, the research team identified issues to consider for further study, the panel then selected two field studies for completion in Phase II: • Operational study on double left­turn lanes. • Operational study on deceleration lanes. The goal of the double left­turn lane operational study was to determine the effects of geometric characteristics on double left­turn lane operations, as measured with saturation flow rate, lane distribution, and driver behaviors. Receiving leg width, left­turn lane width, and downstream friction type and distance were the key geometric variables studied. Iden­ tifying sites with the desired range of receiving leg width was the most difficult of the study variables to satisfy, although finding sites with an average double left­turn lane width greater than 12 ft also proved challenging. Data from 26 sites in three states (i.e., Arizona, California, and Texas) were used in the analyses. The data collection method was video recording. Key findings from the analyses of operations at double left­turn lanes include the following: • A total of 10,023 saturation flow rate values were available for study. The average double left­turn lane saturation flow rate for these 10,023 data points was 1,775 passenger cars per hour of green per lane (pcphgpl). • 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. • The model found that, for each additional U­turning vehicle within the left­turn queue, saturation flow rate would decrease 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 that accounted for a dedicated lane added at the end of a channelized right turn. Although 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. 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 indicates that wider lane widths are associated with higher saturation flow rate, but this study found that the width of the left­turn lanes did not significantly affect saturation flow. This finding could be construed to mean that narrow lanes can be used without affecting operations. In making this interpretation, however,

3 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 left­turn 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 left­turning drivers. For the sites included in this study, receiving leg width ranged from 24 ft to 54 ft, and analysis found that the width affected the saturation flow rate. When the receiving leg width was between 24 and 36 ft, the average saturation flow rate was 1,725 pcphgpl, while a receiving leg width of 40 to 54 ft was associated with an average saturation flow rate of 1,833 pcphgpl. 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 left­turning vehicles, as compared to those described in the Green Book. Researchers collected data from 12 sites in four states (i.e., Alabama, Florida, Mississippi, and Texas). The primary data collection method was video recording, although lidar, global positioning systems (GPS), and traffic counters were used to provide supplemental data. Following data reduction and quality control checks, data from 410 left­turning vehicles were analyzed to investigate three key deceleration lane design guidelines in the Green Book: • The speed differential between turning vehicles and following through vehicles is 10 mph when the turning vehicle clears the through­traffic lane. • The values for deceleration length are based on a 5.8 ft/s2 average deceleration while mov­ ing from the through lane into the left­turn lane. • The values for deceleration length are based on a 6.5 ft/s2 average deceleration after com­ pleting the lateral shift into the left­turn lane. Key findings from the analyses of operations at deceleration lanes were • The Green Book states that a 10­mph differential is acceptable on arterials, but drivers in this study commonly had speed differentials greater than 10 mph, and the higher speed differentials in this study often occurred for vehicles traveling at higher speeds. The Green Book describes the 10­mph differential in relation to a comfortable deceleration for the driver, while support for using 10 mph as the threshold 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 significant 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 8 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 low­speed sites and 6.7 ft/s2 for high­speed sites. In addition, 85% of observed drivers at high­speed sites decelerated at a rate of 4.2 ft/s2 or greater up to the end of the taper. • The deceleration length (at low­speed 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.

4• A design that accommodates decelerating at 4.2 ft/s2 during the lateral movement into the turning lane provides for a more gradual, controlled deceleration, but a higher decel­ eration rate (closer to 6.5 ft/s2 for half of the observed 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, increasing the speed differential between turning and through vehicles. – Some drivers would accomplish more of their deceleration 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 signifi­ cant in determining deceleration rate within the full­width deceleration lane. • Compared to the 6.5 ft/s2 rate noted in the Green Book, approximately two­thirds 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 indicate that a designer could produce a left­turn lane design associated with a deceleration rate of 6.5 ft/s2 and it would accom­ modate the current behavior of 85% of left­turning drivers at high­speed sites and half of drivers at low­speed sites. Researchers developed a list of recommended revisions using the findings from the two field studies, along with the research team’s review of the literature and state design manuals. Key recommended changes are as follows: • Expanding the discussion of bypass lanes in Section 9.3.1, including a cross­reference to warrants suggested for inclusion in a revised Section 9.7.3, based on research in NCHRP Report 745. • Updating the text in multiple sections to better reflect current practice and guidance (including the Highway Design Handbook for Older Drivers and Pedestrians) on skew angle, which should not be less than 75 degrees. • Inserting a new subsection on channelized right­turn lanes in Section 9.6.1 to improve the level of detail found in the Green Book, based on research from NCHRP Project 3­89. • Revising the section on deceleration length and taper length in Section 9.7.2 to incorpo­ rate the findings from this project’s Task 4 deceleration field study. This included adding a subsection on Perception­Reaction Distance, replacing Figure 9­48 and Table 9­22 for clarity and consistency, and better defining the purpose, dimensions, and design of a taper used to add a left­turn lane. • Revising the text in Section 9.7.3 to better describe the capacity benefits and design con­ siderations for multiple left­turn lanes, based on findings from the double left­turn lane operational study in Task 4 of this project. • Making multiple revisions in Sections 9.8 and 9.9 to better describe design features of alter­ native intersection designs, such as U­turn crossovers, median openings, and secondary intersection spacing.