A design for Hoddle Street and Eastern Freeway diverging diamond interchange and how to design two phase intersections

Submitted by john on Mon, 16/12/2019 - 18:48
eastern fwy

Eastern Fwy & Hoddle St A simulation of 2-phase operation is shown on Youtube

The original proposal for Streamlining Hoddle St included 2-phase intersections for the six sites shown here: Eastern Fwy; Victoria St; Johnston St; Wellington Pde; Swan St; and Alexandra Ave. Gipps St was also proposed for 2-phase. Smoothing works were proposed for Truro St, Langridge St, Albert St, Hotham St, Freeman St, Rowena Pde, and Richmond Tce, but they do not add much to the analysis and are not shown. Except that Albert St was the example for smoothing. Connections to the Monash Fwy are considered elsewhere.

VicRoads has reviewed these designs and implemented slightly different concepts at three of the intersections, Eastern Freeway, Johnston Street, and Swan Street/Olympic Boulevard, but has not adopted the green wave concept requiring metering, nor smoothing flow at minor crossings, nor tolled queue jumping. Until the major constriction at Victoria St has 2-phase, the capacity of the route will be unchanged. Current improvements may have inbuilt capacity constrictions at Eastern Fwy and Swan St that will then be tested. Until metering removes congestion, the trip time will be unchanged. Until high priority queue jumping is adopted, extensive queue delay and high CO2 emissions will occur.

The original proposed design has 4 (3) lanes S-E, 3 (2) lanes S-N, & 4 (4) lanes N-S over the interchange structure for a diverging diamond interchange (numbers in brackets are as implemented, but not as a diverging diamond). In contrast, the proposal installed metering, removed congestion, removed the bus lane, smoothed flow, increased capacity, reduced delay by 31 minutes, halved CO2 emissions, and installed queue-jumping for a toll, yet retained free entry via a queue. Pedestrian delays were reduced by providing 60 second signal cycle times. None of these actions were included in the project, and costs were escalated.

Aerial Photography Accurate aerial photography with property boundaries was readily available, printed, trimmed with GIMP software, and scaled as a base for the sketches, using LibreCAD software. A 50% transparency that permits clear vision of the drawing lines was used. VicRoads uses a standard 3m stripe, 9m gap for line-marking, that provides an accurate basis for scaling.

Traffic Volumes Traffic signals recorded volumes for each lane for each ¼ hour. The highest AM and PM peak hours were selected for analysis. Records had the signal phasing and cycle time for the peak hour, that for Hoddle St was 160 seconds. A ½ hour site check was made for both peaks and a manual sample count for each approach made for two cycles and the residual queue lengths observed. The sample counts were taken 3 times in the ½ hour and factored to confirm and detail all movements.

Model Traffic For the AM and PM peak hours, the existing volumes and existing phase times were modelled with Sidra. The approach lane lengths and widths from the aerial photo were used. The model was checked against the observed queue lengths. For the AM and PM peak hours, the 2Pi layout was modelled with existing volumes, and phasing specified without conflicts for the right turns, letting Sidra determine phase times and grow the volumes to at least 150%. Lanes were allocated within the existing footprint using unlimited approach lane lengths and existing lane widths. The required arrangement was sketched and revised to balance the degrees of saturation after recalculation with Sidra. Each of the four approach cross-overs were modelled separately. Within Virtualbox, 150% of the most critical peak was simulated with Vissim to check for erratic vehicle behaviour, and the design reviewed.

Sketch Layout Using LibreCAD and the existing lane widths, parallel lane lines were drawn in the front half of each approach, nearest the intersection, with the right turn lane(s) as a right turn service road. This was repeated with right turns for the back halves in the normal position. The though and turn segments were joined up using tangents and fillets at the design radii. 7m wide splitter islands were provided for both left and right turn slip lanes. Sketching is demonstrated on Youtube

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