Mt Wellington Bottleneck

Options Investigation

The Auckland Southern Motorway narrows from 3 to 2 lanes in each direction across the Mt Wellington overbridge - described as the “Mt Wellington Bottleneck”. This area of the Southern Motorway, a key link in the transport network, suffers from congestion. Many road users perceive the Mt Wellington bottleneck as being a source of congestion, raising the following considerations:

• Is the Bottleneck really a source of congestion?
• Or does the Bottleneck protect other areas of the network from more extensive congestion by metering traffic flow?
• Will widening the overbridge reduce congestion and stack-up economically?
• Are there alternative, notably less extensive, options which bring benefits?

Extensive building and widening of the motorway may be a longer term solution – but what about the short term? What are the answers to these questions, and are there viable less extensive engineering and design solutions?

To explore these concepts we need to measure and predict, robustly, the effects of traffic flow interaction across this motorway system. E.g. the effect of releasing flow on downstream areas, and the effect upstream restricted flow has on the operation and outcome of improvements.

Traffic Design Group undertook a significant data collection exercise, developed a microsimulation traffic model of the wider motorway corridor, and used the model to investigate these questions in some detail.

The model gives the ability to test the theories, assumptions, and concepts – two broad concepts were investigated in detail:

• Road schemes which remove the bottleneck at Mt Wellington by widening the roadway and adding capacity.
• Road schemes / lane markings which maintain similar level of capacity at Mt Wellington but look to rationalise the motorway lane arrangement.

This testing indicated two interesting outcomes:

• Northbound (towards Auckland CBD): widening increases the traffic flow reaching downstream areas of congestion – notably on-ramps and associated merges. This can worsen overall network performance.
• Southbound (away from Auckland CBD): traffic is held in areas upstream of Mt Wellington. This limits the returns offered by widening.

This investigation led to some simple, robust, and viable short term lane arrangement reconfigurations. The recommendations from TDG’s detailed studied have been carried forward into the funding evaluation process for roading projects in this region.