title: Evaluation of left-turn lane offset using the naturalistic driving study data.
abstract: BACKGROUND
  The Strategic Highway Research Program 2 (SHRP 2) Naturalistic Driving Study (NDS) data were used to evaluate gap acceptance behavior of drivers at left-turn lanes with negative, zero, or positive offsets ranging from -29 ft to +6 ft. The objectives of the study were to develop guidance for the design of offset left-turn lanes used as a safety countermeasure, and to provide insight regarding the use of the NDS data to future users.
  
  
  METHODS
  The study included 3350 gaps in opposing traffic evaluated by 145 NDS volunteer drivers and 275 non-NDS drivers at 14 two-way stop-controlled intersections and 44 signalized opposing left-turn pairs. Logistic regression was used to model the critical gap length for drivers as a function of offset, under conditions when their view was either blocked or not by an opposing left-turning driver.
  
  
  RESULTS
  The analysis found that the critical gap was longer at left-turn lanes with negative offsets than at those with zero or positive offsets, and was also longer when sight distance was blocked by an opposing left-turning vehicle. Sight distance was much more likely to be restricted by an opposing left-turning vehicle at negative-offset and drivers at those intersections were less likely to accept a gap when an opposing left-turn driver was present.
  
  
  CONCLUSIONS
  Longer gap lengths could potentially result in decreased operational efficiency of an intersection. In addition, drivers making left-turns at negative-offset left-turn lanes are, on average, more likely to leave the shortest amount of time between their turn and the arrival of the next opposing through-vehicle, which may present a potential safety concern.
  
  
  CONCLUSIONS
  The findings provide guidance for highway designers considering using offset left-turn lanes as a crash countermeasure. This research also highlights the benefits and limitations of using the SHRP 2 NDS data to answer similar research questions.
authors.affiliation: MRIGlobal, 425 Volker Boulevard, Kansas City, MO 64110, USA. Electronic address: jhutton@mriglobal.org.
authors.forename: Jessica M
authors.initials: JM
authors.lastname: Hutton
authors.affiliation: MRIGlobal, 425 Volker Boulevard, Kansas City, MO 64110, USA. Electronic address: kbauer@mriglobal.org.
authors.forename: Karin M
authors.initials: KM
authors.lastname: Bauer
authors.affiliation: MRIGlobal, 425 Volker Boulevard, Kansas City, MO 64110, USA. Electronic address: cfees@mriglobal.org.
authors.forename: Chris A
authors.initials: CA
authors.lastname: Fees
authors.affiliation: Human Factors North, Inc., 174 Spadina Ave, Suite 202, Toronto, ON M5T 2C2, Canada. Electronic address: asmiley@hfn.ca.
authors.forename: Alison
authors.initials: A
authors.lastname: Smiley
mesh_terms: D000063:Accidents, Traffic / Q000517:prevention & control*; D001334:Automobile Driving; D004779:Environment Design; D006801:Humans; D016015:Logistic Models; D018986:Motor Vehicles; D011340:Problem Solving; D012449:Safety
keywords: Accepted gap length; Critical gap; Logistic regression; Offset left-turn lanes; Rejected gap length; Safety surrogates