Geometric Models to Calculate Intersection Sight Distance for Unprotected Left-Turn Traffic

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At signalized intersections with opposing left-turn lanes, inadequate sight distance of left-turn drivers caused by obstructions from opposing left-turn vehicles can not only contribute to a serious safety problem due to drivers misjudging the gaps in the opposing through traffic but also cause increased intersection delay. This delay is created because drivers need more time to make sure that the opposing through lanes are clear. The 2001 AASHTO manual pointed out that the typical poor visibility of opposing through traffic usually occurs at signal intersections with medians wider than 18 ft and recommended two improvement methods: creating parallel offset left-turn lanes and tapered offset left-turn lanes. However, AASHTO did not provide the specific design guideline, nor did it present the related geometric design model. Sight-distance geometric models for unprotected left-turning vehicles from the major road onto the minor road at signalized intersections and vision improvement effects of the two offset methods were developed, and the relationship between available sight distance and related intersection geometric parameters was analyzed.