Method and device for determining demand for external left turn lane at signalized intersection

The invention discloses a method for determining demand for an external left turn lane at a signalized intersection. The method comprises a step of judging a road condition and a timing condition of an intersection, a step of collecting the traffic flow in the direction of each entrance lane of a signal control intersection, a step of judging whether the total traffic flow of left turn and U-turnis larger than a first set threshold, a step of judging whether the ratio of U-turn vehicles to the U-turn vehicles and left turn vehicles is larger than a second set threshold, a step of detecting right turn flow of an intersecting road and judging whether the right turn flow of the intersecting road is larger than a third set threshold, a step of calculating the saturation of a left turn lane with planned external left turn and the saturation of the right turn of a horizontally intersecting road, wherein if one of the saturation of the left turn lane with planned external left turn and the saturation of the right tu