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Analysis of Naturalistic Driving Study Data: Offset Left-Turn Lanes (2014)

Chapter: Chapter 5 - Applications and Recommendations

« Previous: Chapter 4 - Statistical Analysis
Page 56
Suggested Citation:"Chapter 5 - Applications and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Analysis of Naturalistic Driving Study Data: Offset Left-Turn Lanes. Washington, DC: The National Academies Press. doi: 10.17226/22315.
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Page 56
Page 57
Suggested Citation:"Chapter 5 - Applications and Recommendations." National Academies of Sciences, Engineering, and Medicine. 2014. Analysis of Naturalistic Driving Study Data: Offset Left-Turn Lanes. Washington, DC: The National Academies Press. doi: 10.17226/22315.
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Page 57

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56 C h a p t e r 5 applications The results of this research are applicable to highway design- ers and traffic engineers who are designing new intersections with uncontrolled approaches or permissive left-turn signal phasing, or who are considering safety countermeasures for demonstrated or anticipated crashes at intersections. The critical gap times found in this research ranged from 4.7 s at intersections with the most positive offset to 7.5 s at intersections with the most negative offset. Sight-distance restrictions due to the presence of opposing left-turn vehicles, which were much more common at left-turn lanes with nega- tive offsets, also led to longer critical gap lengths. The criti- cal gaps observed in the research are closely aligned with the critical gap lengths reported in other studies. The AASHTO Green Book recommends a critical gap of 5.5 s for passenger cars on an undivided two-lane highway, with an additional half second added for each additional lane crossed by the turning driver (AASHTO 2011). The HCM has used a critical gap length of 4.5 s for signalized intersections with a permis- sive left-turn phase (Transportation Research Board 2000). The results of this study suggest that the critical gap length used for design guidance and operational analysis might be less than observed critical gaps, especially at intersections with negative-offset left-turn lanes. While differences in critical gap were not significant between each offset category, the data clearly showed that negative-offset left-turn lanes have longer critical gaps than positive-offset left- turn lanes, and that opposing left-turning vehicles are much more likely to cause a sight restriction at negative-offset left- turn lanes than at positive-offset left-turn lanes. Therefore, even opposing left-turn lanes with a minimally negative offset (-1 ft to -5 ft) have the potential to benefit from relocating the left-turn lanes to a positive offset. In addition, the data did not show that offsetting the left-turn lanes to make them less negative (e.g., changing an offset from -16 ft to -6 ft) would have a substantial effect on safety or operational concerns. In other words, offsetting left-turn lanes will provide operational and safety benefits only if the left-turn lanes are relocated suf- ficiently to provide at least a zero offset, but preferably a posi- tive offset. The degree to which a left-turning driver’s view of oncom- ing traffic is restricted by the presence of an opposing left- turn vehicle depends on several factors, including, but not limited to, the offset of the left-turn lanes. The geometry of the intersection, especially the width of the roadway being crossed, determines how close the opposing left-turn vehicles are to each other and to what extent they block each other’s driver’s view of oncoming traffic. In addition, the geometry and striping of the intersection likely play an important role in allowing or encouraging turning drivers to best position themselves for optimal viewing of oncoming traffic without encroaching into the opposing lanes. Therefore, engineers must look at the complete design of the intersection, con- sidering the positioning of left-turning vehicles waiting for a gap, to assess the degree to which sight limitations caused by opposing left-turn vehicles might be an issue. The analysis conducted as part of this research indicated that even at intersections with left-turn lanes that have only a small negative offset (-1 to -5 ft), the presence of opposing left-turn vehicles can restrict sight distance for their respec- tive drivers. In addition, at negative-offset left-turn lanes, driv- ers are more likely to leave a shorter amount of time between their left-turn maneuver and the arrival of the next opposing through vehicle. This indicates that any degree of negative off- set might create a safety concern for left-turning drivers. The Green Book suggests that offsetting the left-turn lanes should be considered at intersections with medians wider than 18 ft (which most likely corresponds to an offset of approximately -6 ft). That may be somewhat arbitrary and not conserva- tive enough. This research indicates that eliminating negative offset to provide positive-offset (or, at minimum, zero- offset) left-turn lanes should be considered to substantially reduce instances of sight-distance restriction for left-turning Applications and Recommendations

57 drivers where feasible and practical. When potential sight- distance issues cannot be avoided, designers should consider protected-only left-turn signal phasing to eliminate the need for drivers to judge gaps in opposing traffic. In general, the secondary analyses discussed in Chapter 4 did not show meaningful relationships or conclusive results because of limited sample size. The NDS data and the study design used have potential to show a wide range of relation- ships and results considering a number of possible influences on left-turn behavior; however, the limited data available at the time data requests were made (VTTI estimated that only 20% of the trips that will ultimately be included in the NDS database were available to be queried), in combination with the short schedule of this project, constrained the sample sizes the research team was able to obtain. Obtaining and reviewing another round of videos of left-turning movements made at the study intersections in this report would potentially show more meaningful results. In addition, the NDS data allow researchers to identify specific factors that may influence turn- ing behavior and query for a distribution of those specific char- acteristics on which to conduct an analysis. recommendations for Future research While this research presents a range of analyses related to left-turning behavior at intersections with varying offsets, turning behavior can be evaluated in a number of ways. The NDS data could be used in future research to consider the following: 1. Additional measures of interest could be collected, such as the duration of the turning maneuver, the distance traveled to complete the turning maneuver, and the distance of stop bars from the conflict area. When sample sizes are large enough, the effect of these measures on where drivers posi- tion themselves before turns and how their positioning affects their sight distance and time required to turn could provide valuable information to intersection designers and traffic engineers. 2. The NDS data provide an opportunity to follow a single driver through many left-turn maneuvers. The factors that influence a single driver’s differences in behavior from one intersection or circumstance to the next can be identified. 3. The NDS data include information about vehicle dynam- ics. The influence on turning behavior of a vehicle’s physi- cal turning capabilities, as well as the speed and acceleration used when approaching and during the turning maneuver, could be evaluated to provide guidance to vehicle manu- facturers on designing vehicles for safe left turns. 4. Crash history could be used in the study site selection pro- cess to specifically identify intersections with similar geo- metric characteristics but different crash rates to explore how drivers’ behaviors differ between intersections and what features or conditions might be influencing those behaviors. 5. Left-turning behavior could be evaluated in specific con- ditions, such as hours of darkness or during rain events. The NDS data allow for such queries so researchers can ensure desired sample sizes of more rare events. 6. The effect of left-turn lane offset on intersection opera- tional performance (in terms of delay or level of service measures) could be evaluated. Such an analysis could identify an operational benefit–cost ratio for implement- ing a left-turn lane offset countermeasure.

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TRB’s second Strategic Highway Research Program (SHRP 2) Report S2-S08B-RW-1: Analysis of Naturalistic Driving Study Data: Offset Left-Turn Lanes evaluates the gap acceptance behavior of drivers at left-turn lanes with offsets ranging from -29 feet to 6 feet.

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