Diverging Diamond Interchanges have more capacity and fewer crashes than other interchanges

Submitted by john on Sun, 15/12/2019 - 19:20
eastern hoddle ddi

A Diverging Diamond Interchange is where arterial flows cross to the wrong side of the road, over the structure, and then cross back again. This is the preferred interchange option. This enables flows from both directions to enter the entry ramps by simply diverging without further signal control, except for ramp metering and pedestrian crossings, and also allows flows from the exit ramps to enter the arterial road, signalised in parallel with one of the arterial road through flows, or by added lanes. Pedestrians and cyclists should normally cross the structure in the centre, as shown here, between protective barriers.
This sketch is overlaid on the Eastern Fwy, Hoddle St interchange and is a pair of 2-phase intersections. North is rotated to the right. Note the quite short clearance distances. A simulation of the Eastern Fwy, Hoddle St DDI option is shown on Youtube.

Where there are ramp meters, the meter can be at the top of the ramp, with queues extending through the structure. Ramp meters limit the flow of traffic to the freeway to avoid flow breakdown. Meters at the top of the ramp reduce the required ramp length, and increase the distance to the next freeway exit, so reducing freeway turbulence.
The video for Marvin Rd interchange in Washington State notes 60 DDI’s built since 2009, having greater capacity, with 50% crash reduction, and lower crash severity.

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