Evaluation of left-turn lane offset using the naturalistic driving study data.

Area of Science:

• Transportation Engineering
• Traffic Safety Research
• Human Factors in Driving

Background:

• Investigates driver gap acceptance at offset left-turn lanes using Strategic Highway Research Program 2 (SHRP 2) Naturalistic Driving Study (NDS) data.
• Evaluates negative, zero, and positive offsets (-29 ft to +6 ft) to inform safety countermeasure design.

Purpose of the Study:

• Develop design guidance for offset left-turn lanes as a safety countermeasure.
• Provide insights into utilizing NDS data for future traffic safety research.

Main Methods:

• Analyzed 3350 gaps at 14 two-way stop-controlled and 44 signalized intersections.
• Employed logistic regression to model critical gap length based on offset and sightline obstruction (blocked vs. unblocked).
• Included data from 145 NDS volunteer drivers and 275 non-NDS drivers.

Main Results:

• Critical gap lengths were significantly longer for negative-offset lanes compared to zero or positive offsets.
• Blocked sight distances, common at negative offsets, further increased the required critical gap.
• Drivers at negative-offset locations were less likely to accept gaps when an opposing left-turning vehicle was present.

Conclusions:

• Negative-offset left-turn lanes may decrease operational efficiency due to longer required gaps.
• Potential safety concerns arise from drivers at negative-offset lanes accepting smaller time intervals before opposing vehicles.
• Findings offer guidance for highway designers on using offset left-turn lanes for crash reduction.