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Traffic Signal Systems Operations and Design: Isolated Intersections
3. Based on your reading, would you change the left turn phasing plan that you developed in Activity
#50? Explain your answer.
by Tom Urbanik
Two issues have a profound effect on left turn operation and therefore phasing considerations. First is
available storage which may be constrained due to closely spaced intersections. If the cycle length is
long and the volume is high, consideration may need to be given to running the left turn twice per cycle.
While one might think it is less efficient with two clearance intervals per cycle, a green indication with
through traffic blocking the left turning traffic or left turning traffic blocking through traffic is more
inefficient.
The second issue is when does left turning traffic arrive at the left turn lane. Again, with closely spaced
intersections, left turning traffic may arrive too late to be served on the current cycle, causing increased
delay for left turning traffic. Lagging rather than leading the left turn phase may provide reduced delay.
This situation is very common at diamond interchanges where lagging the left turn to the ramp is often
the preferred sequence.
A
ctivity
51: L
eft
T
urn
P
hasing
O
ptions