Describe the effects of the three left-turn treatments at signalized intersections on saturation flow rate and geometric design.

Question

What will be an ideal response?

Answer 1

Left-turn vehicles at signalized intersections can proceed under one of
three signal conditions: permitted, protected, and protected/permissive turning
movements.
Permitted turning movements are those made within gaps of an opposing
traffic stream or through a conflicting pedestrian flow and should yield to
conflicting traffic and pedestrian movements. Permitted operation is primarily
used when traffic is light to moderate and sight distance is adequate. This mode
can have an adverse affect on safety in some situations, such as when the left-turn
driver's view of conflicting traffic is restricted or when adequate gaps in traffic are
not present.
Protected turns are those turns protected from any conflicts with vehicles
in an opposing stream or pedestrians on a conflicting crosswalk. All conflicting
movements must therefore yield to protected turns. A permitted turn takes more
time than a similar protected turn and will use more of the available green time.
This operation provides for efficient left-turn movement service; however, the
added left-turn phase increases the lost time within the cycle length and may
increase delay to the other movements. An exclusive left-turn lane is typically
provided with this phasing. The left-turn phase is indicated by a green arrow
signal indication. This type operation is recognized to provide the safest left-turn
operation.
Protected-Permitted is a combination of the protected and permissive
conditions, in which vehicles are first allowed to make left turns under the
protected condition and then allowed to make left turns under the permissive
condition. This mode provides for efficient left-turn movement service, often
without causing a significant increase in delay to other movements. This mode
also tends to provide a relatively safe left-turn operation, provided that adequate
sight distance is available and turns during the permissive component can be
safely completed.
The determination of the specific treatment at a location depends on the
transportation jurisdiction as guidelines vary from one jurisdiction to another. A
chart proposed in the FHWA Traffic Signal Timing Manual can be used to justify
the necessity of a protected left-turn phase.