Improving roads by design at Australia’s gateway

Sydney Airport is one of Australia’s most important pieces of infrastructure and is a critical economic engine for Sydney, NSW and Australia. Directly and indirectly, the airport facilitates the equivalent of almost seven per cent of the state’s economic activity and 338,500 jobs. The passenger forecasts for the airport presented in the Master Plan 2039 indicate growth from 43.3 million passengers in 2017 to 65.6 million passengers in 2039.

In recent years, the need to address the growing volumes of traffic on major roads approaching Sydney Airport, as well as the traffic on roads within the airport’s international and domestic precincts, has become paramount.

Making it easier for motorists to travel to, from and past the airport by easing traffic flows and reducing congestion has been a key priority for Sydney Airport since at least the early part of the decade, as has the airport’s commitment to providing the widest possible range of transport options for travellers and farewellers.

In 2014, Sydney Airport received Federal and State Government approval to begin work on one of its most ambitious ground transport infrastructure projects to date, namely to design and construct a radically modified road network within the terminal precincts.

This involved a variety of major challenges; not least of these was the need to design and build new roads within the constraints of a comparatively small airport campus (Sydney Airport covers just 910 hectares) that is visited by 150,000 people every day; and to maintain the efficient ground transport operations of a major piece of infrastructure 24 hours a day.

This complex task involved both optimisation of existing infrastructure, and introducing improvements and upgrades by developing new infrastructure.

Upgrading the road network

When work on the design of the new road network began in 2014, the airport’s T1 international precinct in particular was under increasing pressure from growing traffic volumes. Traffic entering T1 via Marsh Street (from the south and west) and via Airport Drive (from the north and east) was generally accessing three areas within the airport: the departure drop-off deck, the arrivals area (hotel, office buildings and bus/coach parking) and the car parks (multi-level and open on grade).

With traffic entering the airport, one of the main challenges was that traffic coming from two directions (Marsh Street and Airport Drive) was converging into one road, and then diverging to the three main destinations within the airport (departures, arrivals and car park). This short section of single road was experiencing multiple, conflicting, vehicular weaving movements that, under increased traffic volumes, resulted in compromised road safety, congestion and delays.

Sydney Airport engaged Mott MacDonald to develop first the concept design and subsequently detailed design and construction support for an overall solution that untangled the conflicting movements, significantly increasing road safety and traffic capacity, and reducing congestion and delays.

Known as Project 2B and Project 3, the designs addressed the traffic entering from Marsh Street and Airport Drive respectively. The essence of the solution was to provide as direct access as possible for Marsh Street and Airport Drive traffic to each of the three destination areas within the airport by substantially decreasing or eliminating the conflicting and unsafe weaving movements.

Under Project 2B, Bridge Crescent (the entrance road from Marsh Street) was widened by an extra lane to provide extra capacity for three-way distribution, before conflicting with Airport Drive traffic. An important new connection was added to Centre Road to provide direct entry to the car park, which meant a significant reduction of weaving traffic. The function of the recirculation lane was fully maintained and improved in terms of sight distance and safety.

Project 3 provided another connection that bypassed the conflicting weaving area and enabled direct access for Airport Drive traffic connecting to the arrivals area.

Completed six months ahead of schedule in June 2018, these two new connections have largely eliminated traffic weaving movements and have substantially reduced the traffic on the existing road. As a result, increased vehicle capacity and a vastly improved flow of traffic along the entry system has led to a better customer experience for both private and commercial motorists, and ease traffic congestion along both Marsh Street and Airport Drive.

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Specific design considerations

Given the severe constraints of the program, the design of Projects 2B and 3 required the close collaboration of a large and specialist multidisciplinary team.

Sydney Airport and Mott MacDonald assembled a team of highly experienced project managers, engineers and designers — with expertise in road geometry, traffic engineering, stormwater drainage, structural engineering, geotechnical, pavement and utilities management — who produced the design that was used to construct the works.

Road geometry design was very constrained due to the limited space and existing roads and viaducts in the vicinity. The new connections on Project 2B and the new overpass on Project 3 were challenging as they stretched the design parameters in order to provide the required clearances, offsets and sightlines. Also, levels were optimised to preserve and utilise the existing pavement, while allowing for extra strengthening where required.

Stormwater drainage was constrained in terms of discharge levels due to the close proximity of the works to the bay. Therefore, the preference was to adjust the design (both in position and level), so the existing stormwater pit and pipe system was used wherever possible and this, in turn, influenced the road geometry.

From the structural design point of view, the new overpass viaduct design required a lot of attention to determine the most appropriate type of structure — the central span over Bridge Crescent was carefully selected to enable quick positioning and minimise the impact on traffic. There were also several low-height walls that were incorporated in safety barriers to optimise the limited space.

Challenging construction conditions

An overarching challenge on both projects was the difficult ground conditions, combined with the relatively high underground water table. The geotechnical team had to carefully consider treatments of the new viaduct supports and footings, including the abutments. The analysis of the ground conditions was also very important when designing the road pavement structure.

The work was made more complex by the presence of underground utilities. The design was coordinated in detail with the existing utilities including water mains, water tanks, high-voltage electricity lines, etc, to minimise impact and, where this was not possible, to determine the most effective and most optimal relocation.

With every new or rearranged traffic system, it is of high importance to develop new directional signposting (wayfinding) so motorists can be informed and directed. To this end, structures were designed to support several new and large digital VMS (variable message signs) that inform motorists of any changes in driving conditions and directions.

Sydney Airport and Mott MacDonald continued to work closely throughout the construction stages, with joint site inspections and site meetings, modifying and adjusting the design to respond to any construction challenges and issues.

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