Left-turning movements affect considerably the capacity of traffic signal controlled intersections, especially, when there is no separate left-turn lane and vehicles wishing to turn left must share the same lane with straight ahead traffic and the same green stage with an opposing flow. In such a case the capacity is influenced by a number of traffic and geometrical parameters. As flow levels and experience have increased over the years, methods and means have been developed to cope with the growing traffic demand. As volumes increase, the need for protecting left-turners increases also. The models are based on a diversity of assumptions and each has some limitations when applied in practice. In addition, all models, but a few, don't account for possible spaces existing within the intersection area for left turner to be stored without blocking straight ahead vehicles sharing the same lane. This paper presents a probabilistic model developed for calculating the capacity of shared left-turn lanes with an opposing traffic flow at traffic signals controlled intersections. It was validated by field data collected at six sites with 1, to 5 spaces available for left-turners seeking suitable gaps in the opposing flow without blocking the progression of straight through vehicles.
