Ramping up traffic flow

The installation of ramp signals on the Southern Motorway has proven to be a very effective part of a number of initiatives to improve congestion, safety and travel times on Auckland’s motorway system.   BY MARY SEARLE

Ramp_1.jpgAuckland’s motorway system is a complex beast, it stretches across the city in three directions, twisting together at Spaghetti Junction.  Along its length many feeder ramps fill it daily with thousands of vehicles from the city’s streets.

Managing the traffic on the motorway to ensure it keeps flowing at a reasonable pace, especially at peak hour, is an important part of the NZ Transport Agency’s function. To achieve this, it has established a motorway management system in collaboration with all of the region’s city and district councils – Auckland City, Auckland Regional, Manukau, Papakura, North Shore, Waitakere, Franklin and Rodney – and the Auckland Regional Transport Authority.

NZTA travel demand management project director Peter McCombs says the project purpose is to improve the network management, deal with congestion, provide consistent travel times, reduce user costs and emissions, and provide better safety and easier travel.

“This can only be done if everyone does it together,” he says.

One of the key tools in achieving this is the implementation of ramp signals along the motorway’s length.

What are ramp signals and how do they work?

Ramp_2.jpgMost accidents on Auckland motorways happen during peak hours when traffic is stop-start, due to lapses in driver concentration and motorists travelling down crowded on-ramps vying for positions in traffic or trying to merge together.

Ramp signals are traffic lights at the top of motorway on-ramps that manage the flow of traffic onto the motorway during peak periods. With each green light, two cars (one from each lane) are able to drive down the ramp to merge easily, one at a time, with motorway traffic. Vehicles move forward every few seconds between green lights. Separating the vehicles travelling down the on-ramp makes merging onto the motorway easier, minimises stop-start conditions and causes less disruption to motorway traffic flow.

Ramp signals operate according to traffic flow, with electronic sensors, built into the road, detecting when traffic becomes heavy. A message is then sent via fibre optic cables to an automated server which immediately switches the ramp signals on.

The signals manage the flow of traffic onto the motorway during peak periods and other busy times. The signals only operate when needed to improve traffic flows. At other times they remain off.

The system is made up of a number of traffic detectors and associated algorithms that detect when queues are getting longer and changes the signal phasing to speed up traffic through the on-ramp. The system will persist as long as it can help traffic flow. When traffic is so busy it can’t help, or so quiet it doesn’t need assistance, the system will turn off, automatically turning on again when it can help.

Depending on location, additional cameras and detectors are installed on the approach to the on-ramp to measure the ramp and local road traffic volumes – although motorway traffic is allowed to queue on suburban roads, it can’t impinge on non-motorway traffic. NZTA operators have a range of cameras giving a continuous view of traffic operations in the area.

What’s been done so far?

Ramp_3.jpgThe project got underway in 2006 with a ramp signal trial on State Highway 20 Rimu Road on-ramp. The positive result saw Transfield Services install ramp signals on all 31 on-ramps on the Southern Motorway and Central Motorway Junction. Another 15 are currently being installed on the Northern Motorway – due for completion in February next year, and 15 on the Northwestern Motorway – due for completion in June 2009. Ramp signals will also be installed on the Southwestern Motorway.

Compliance with the ramp signals is very good – in fact it’s considerably better than at Auckland intersections where running red lights is a real problem. Data shows only four to seven non-compliances per 50 cycles at ramp signals – a non compliance is when an extra car sneaks through on the green or cars run an amber or red light – whereas a two-lane intersection in the city will have at least one non-compliance per cycle.

Priority lanes

Truck priority lanes allow trucks to bypass the ramp signal without stopping and losing momentum, especially when on-ramps are on an incline. The faster access means that trucks can maintain a good speed onto the motorway, and will not slow down other vehicles.

The truck priority lanes can also be used by buses, taxis and car-pool vehicles (two or more people per car) – moving more people rather than more cars.

A total of 14 selected on-ramps across the Southern, Northern and Northwestern motorways will feature priority lanes.

How effective are ramp signals?

Peter McCombs says the ramp signals are achieving what they were installed to do – increase throughput and travel speeds.

For example, the Wellington Street northbound and Northwestern-Port to North ramp signals manage the key Victoria Park Viaduct two-lane bottleneck. Since they’ve been installed an extra 230 vehicles per hours are getting through the bottleneck (a throughput increase of six percent), and the vehicles are travelling about six kilometres an hour faster (up 4.5 percent).

Another example is the Curran Street northbound ramp signals, which feeds onto the Auckland Harbour Bridge. There was a lot of objection from local residents when these signals were due to be installed, who were worried about vehicles, especially trucks, queuing up on the suburban streets, waiting to get on the motorway. Now that the ramp signals are up and working the only compliant from local residents is that the ramp signalling should have been done before, says McCombs.

He says the Curran Street ramps signal has delayed the onset of congestion on the Harbour Bridge by 30 minutes, the average travel speed is 13 kilometres an hour faster and peak throughput has increased by 18 percent – an extra 180 vehicles an hour.

The heaviest load on the entire roading network is the piece of motorway southbound between Hobson Street and Market Road. Since introducing ramp signals on the two on-ramps on this stretch throughput during peak times has increased 15 percent – that’s an extra 600-800 vehicles per hour – and individual ramp volumes have increased by 150 vehicles per hour. The busiest piece of road in the country now routinely carries 6750 vehicles an hour in three lanes with an improved travel speed of 10km/h and homeward evening commuter traffic is clearing 30 minutes earlier.

What’s next?

A number of new initiatives are being introduced to improve the network management. One currently being worked on is traveller information signs – NZTA plans to install 300 of these along the network to display things like road conditions ahead, expected journey times to various destinations, etc.

Another initiative being worked on is a performance metrics project that will provide information about the motorway in real time on the web and cellphone updates to subscribers.

The NZTA is also working to provide more extensive data on the network to interested parties – things like traffic volumes, speeds, weather conditions and travel times. 

Contractor Vol.32  No.10  November 2008
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