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Chapter 8: Left Turn Phasing—Permitted, Protected, or Both
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The purpose of this activity is to give you the opportunity to compare permitted and protected left turn
phasing.
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Show that protected left turn phasing is more efficient than permitted left turn phasing under some
conditions
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Movie file: A48.wmv
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Prepare a document that includes your answer to the Critical Thinking Question
As you begin this activity, consider the following question. You will come back to this question when you
have completed your observations.
1.
How does changing from permitted to protected left turn phasing affect the left turn operation and the
operation of the entire intersection?
In the previous activity, you considered the efficiency of permitted left turn operations. You saw that
high opposing through volumes could seriously degrade the quality of permitted left turn operations.
One option to improve the left turn operation is to change the phasing from “permitted” to “protected.”
Figure 166 illustrates the ring barrier diagram for full left turn protection.
In this activity you will again observe the left turn operation on State Highway 8. Both cases that you
will observe have through volumes of 1450 vehicles per hour per lane and left turn volumes of 100
vehicles per hour. The only difference is in the left turn phasing. Case 1 is permitted left turn phasing
(similar to case 2 in the previous activity) while case 2 is protected left turn phasing.
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Comparing Permitted and Protected Left Turn Phasing