Page 268 - Book_traffic_e

268 Traffic Signal Systems Operations and Design: Isolated Intersections
4. Prepare a brief summary of the performance of the left turn movements for each case. Consider the
relative size of the queues that form and the relative delay experienced by the left turn movements.
In this activity you will observe the operation of State Highway 8 and Line Street, focusing on the left
turn operations on State Highway 8. An aerial view of the intersection is shown in Figure 163. State
Highway 8 has two through lanes in each direction, while Line Street has one through lane in each
direction.
The left turn phasing that you will observe is called “permitted,” since the left turn traffic is allowed or
permitted to complete their turning maneuver only if there is a safe or acceptable gap in the opposing
through traffic. If an inadequate number of gaps in the opposing through traffic present themselves,
the quality of the left turn operations will deteriorate.
Two cases will be considered here, each with different opposing through volumes. In the first case,
the opposing through movement is 800 vehicles per hour. In the second case, the opposing through
movement is 1450 vehicles per hour. In both cases, the left turn movements are 100 vehicles per
hour. The minor street movements (northbound and southbound through movements) have the same
volume, 600 vehicles per hour.
Figure 163. Aerial photograph, State Highway 8 and Line Street
Activity 47: Permitted Left Turn Operations