Indirect turns are inefficient and confusing

Submitted by john on Sun, 15/12/2019 - 19:05
Pturn

(b) P-turn
Right turners in a P-turn: go through, perform a U-turn and then a left turn, as is shown in magenta on this sketch of Victoria St and Hoddle St. P-turn is to be avoided because it is unusual; needs to be signed in advance; leaves open the option for people to turn right, illegally and unsafely, where they conventionally do; and incurs extra travel distance.

(c) Q-turn
Right turners in a Q-turn: turn left, then perform a U-turn, as is shown in green on this sketch of Victoria St and Hoddle St. Q-turn is to be avoided because it is unusual; must be performed from the left; needs to be signed in advance; and incurs extra travel distance. A Q-turn has been used at Moorooduc Hwy, Cranbourne Rd intersection.

Regarding this sketch, having 2-phase intersections, 2Pi, is critical for capacity and delay. On the south approach, the 100 vehicles per hour right turn lane is replaced with a Q turn, shown green, to gain an extra through lane within a restricted footprint, a gain of 1,000vph. On the east approach, the right turn is replaced with a P turn, shown magenta, to prevent queuing on the tram track. These P-turn and Q-turn options with low volumes increase capacity even though traffic re-enters the intersection.
The option with the highest practical capacity, for the target pattern of traffic demand, will reduce delays. The existing layout for this intersection has a practical capacity of 2,289vph southbound; it is currently operated highly congested at 3,341vph; and the 2Pi design sketched has a practical capacity of 5,240vph. Sidra was used to calculate the practical capacities. The existing flow was counted by the traffic signals.

The simulation of the Victoria St, Hoddle St intersection includes the P-turn and Q-turn on the sketch.
While in theory, only two U-turns are needed to process four right turns at an intersection, and the example shows one U-turn serving two right turns, this should be avoided because it is not driver friendly.

(d) Parallel Flow
Parallel flow, is a right turn from the conventional location, but into a near-side service road, during the side road phase, crossing back to the normal lanes some distance down the departure leg. This option uses right turn arrows for the main road when the cross traffic is running. A presentation of parallel flow is on Youtube.
This option is inferior to the previous three options. Either the cross back to the normal location interrupts both directions so can’t be linked both ways to the main intersection, or the cross back is to an additional median lane, that then merges. Both require yet another lane for the left turn from the side road being entered that must diverge before the cross back. Requiring additional lanes within a restricted footprint seriously reduces capacity.

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