Increase through traffic capacity by 70% and halve crashes by processing right turns on approaches, concurrent with cross traffic

Submitted by john on Sun, 15/12/2019 - 16:34
wasted time

( In US: Increase through traffic capacity by 70% and halve crashes by processing left turns on approaches, concurrent with cross traffic.)

Increasing capacity improves the quality of service by reducing the delay at an intersection queue. In fully developed urban areas, it can be quite disruptive to acquire property to widen the road reserve. So it is highly desirable to get the most traffic capacity out of the existing number of traffic lanes. Approaches to major intersections often have short additional lanes, known as flaring, but to match the mid-block capacity, that would need to double the number of through lanes, and to have full utilisation of those extra lanes. Some flaring lanes are usually reserved to safeguard turning traffic, with its differential speed.

Two-Phase Intersections
In addition to flaring, a good strategy is to process through traffic for a longer proportion of time at the intersection, and to resolve right turn conflicts with the opposing through traffic on the approaches, within the flaring, timed to coincide with the cross traffic. This usually requires an additional set of traffic signals on each approach, but they are easily justified on the basis of a 70% increase in capacity, safer operation, lower delays, avoidance of property acquisition, better lane utilisation, and less turbulence.

“Continuous flow” intersections are used in Utah on the Bangerter Highway. They have crossovers on the main approaches to the intersections, but not on the side road approaches. Federal funding is limited to the highway approaches. Turn conflicts with opposing traffic are resolved on the main approaches with a “crossover”, saving time lost at the intersection due to one turn phase. These intersections have ~35% improvement in capacity and delay and ~50% improvement in safety. Cross-overs are presented shortly. “Continuous flow” intersections are dimensioned so that the cross-over is located the distance from the cross road such that the trip time for through traffic from cross road to the cross-over plus the time for the right turner from cross-over to the cross road exceeds the non-green time so that the right turner has continuous flow. This places the cross-over ~200m from the cross road. This is unsuitable for the urban context with access to properties; for short flaring; does not have the option of multiple cross-overs; gives undue importance to the right turns; some drivers miss their turns; and the proposed two-phase intersections do not have these problems.

Common sense will tell you that the delays caused by turn conflicts from the side road are of similar size and should also be removed. This can be done within the existing footprint, and it is more driver friendly if all approaches are the same. There are different ways to resolve the right turn conflict, yet the essential requirement is for the intersection to have only two signal phases, and it should be known as such, a “two-phase” intersection.

The figure "Wasting Time" compares the allocation of time for the conventional 4-phase and the proposed 2-phase intersections, both with 120 second cycles. In practice, 4-phase intersections often need 120 second cycle times in order to limit the proportion of lost time: amber and all red. Longer cycle time increases capacity at the expense of delay. With 2-phase intersections they have no turn phases at the intersection, and capacity is increased by 69% for 120 second cycles, calculated as 90% /53%. But for the current proportion of lost time, 2-phase intersections should normally operate with 60 second cycles with only 51% more capacity and importantly, much less delay. Maximum delay for through cars is then one short phase of 36 seconds for 2-phase intersections, instead of 3 longer phases for 94 seconds for 4-phase intersections. The option of operating the intersection at 120 seconds cycles is retained.

A simulation of the Victoria St, Hoddle St intersection is shown on Youtube using 5,240vph southbound. It has a green wave, no residual queues and carries 50% more than the existing congested intersection that has a volume of only 3,341vph southbound and 4km of queue, extending past four more intersections and on to the Eastern Freeway. If queue jumping is permitted, average trip time is then 9 minutes instead of the current 40 minutes. Queue jumping is presented later.

Complex intersections with more than four legs can, and should be converted to four legs, where 2-phase intersections can be applied. A simulation of a 2-phase intersection with six approaches at Princes Hwy, Springvale Rd, Centre Rd is on Youtube, although this intersection warrants a grade separation. The safety and capacity benefits of processing right turns on approaches should also apply for low volume un-signalised intersections.

Safety
Instead of sitting on the approach, doing nothing while the cross traffic runs, the right turners perform a “get-out-of-the-way” movement, by crossing the path of the opposing through traffic, thereby operating in parallel with the intersection cross traffic and avoiding the delay of a separate turn phase at the intersection. Separating these right turn conflicts away from the intersection simplifies driving decisions, and has an enormous benefit for safety, with only half the crashes, so all conventional intersections are now relatively dangerous, and should no longer be built on the bases of safety, capacity and delay. Existing intersections, particularly those operating in excess of 90% capacity, can be converted to 2-phase, within the existing footprint with capacity and delay benefits as well as safety.

Comparisons
Two phase intersections here do not relate to intersections where right turns filter through opposing traffic as this is dangerous. Two phase intersections are also called parallel flow intersections, continuous flow intersections, displaced right turns, or displaced left turn, and the descriptions are often inconsistent for the concept being used. Further, in contrast to the conventional bias towards increasing the footprint size, all the deigns in this web site use the existing footprint, existing number of lanes and lane widths, and still achieve 70% increase in capacity with better alignments than existing, and increased safety. Note that the option of reducing signal cycle time from 160 seconds to 60 to reduce pedestrian delay is preferred, leading to only 50% capacity increase. Pedestrian safety is also increased by full signalisation, no filtering through pedestrians, and fewer and shorter pedestrian crossings.

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