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posted on 13.04.2021, 17:50 by Ray Saeidi RazaviRay Saeidi Razavi, Peter FurthPeter Furth

At signalized intersections, permitted left turns (i.e., on a green ball, after yielding) across multiple through lanes and across a separated bike lane or bike path present a substantial threat to bicyclist safety. A conflict study of two such intersections found that when bicyclists cross while one or more vehicles is waiting to turn left and there is no opposing through traffic to block it, the chance of a motorist yielding safely (i.e., waiting in the left turn lane) was only 9%, and the chance of their yielding at all – including yielding only after beginning the turn, then stopping in the opposing through lanes – was still only 37%. Non-yielding rates were worse for bicyclists arriving during green, for bicyclists on a two-way bike path riding on the right side of the road and therefore facing a left cross conflict, and for bicyclists facing a queue with multiple left turning vehicles. Of 112 cyclists who arrived on green when there was at least one left-turning car but no opposing through traffic blocking it, 73 had to slow or stop to avoid a collision. While these conflicts could be essentially eliminated using protected-only left turn phasing (turn on green arrow), existing criteria prefer permitted left turns to reduce vehicular delay. A case study shows how, by considering multiple signalization alternatives, it can be possible to convert left turns to protected-only phasing without imposing a substantial delay burden on vehicles or other road users.

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