Shelley Bridgeman: The best and worst of Auckland's motorways

Motorway traffic grinds to a halt after a crash in Te Atatu. Photo / Dean Purcell

Thanks to frequent journeys along the southern motorway, I have firsthand experience of Auckland's notoriously fickle motorway system.

Luckily I'm mostly able to plan my travels to avoid peak times; heading north in the early morning rush-hour or south in the evening crush would very likely do my head in.

Recently I've been pondering the merits of the on-ramp innovations that were introduced a few years ago to smooth traffic. I'm not a huge fan of the traffic signals allowing one car per lane to proceed each time the light turns green but I have grown fond of those electronic light boards that show estimated journey times.

Red-green light signals

The purpose of these is to ease congestion on the motorway itself by drip-feeding vehicles through the on-ramps. But there's criticism that they actually cause a build-up of traffic that then banks up through suburban streets. The motorways might run more smoothly but feeder streets become more hazardous at peak times.

These lights invite an unofficial drag race. Surely I can't be the only motorist to quietly eye up the opposing vehicle to figure out how much gas I'll need to get ahead. If your competitor is a parent in a station wagon or a tradesperson in a van, it won't be difficult to emerge the victor.

On the other hand, if your fellow road-user looks like a formidable opponent you may decide to be gracious and allow her (or him) to get away first. Most often, though, I'm in it to win it. Plotting a speedy getaway is one way of alleviating the boredom of sitting in line and staring at a red light.

I love it, though, when the vehicle immediately ahead of you jumps the gun and departs while the light is still red. Then suddenly the light goes green and, hey presto, you're allowed to get away earlier than expected. Whether you call it a free pass or an early Christmas present, it's a gift bestowed thanks to the impatience of the driver in front of you.

Estimated journey time signals

I like those estimates that give an indication of how heavy the traffic is between where you are and where you want to go. When I get on the north-bound southern motorway at Hill Road, Manurewa, I check the estimated travel time to Greenlane. Fourteen minutes is the lowest it has ever been; I love it when it says fourteen minutes. Once it said 52 minutes; I did not love that at all.

For a couple of weeks in December, I took note of the estimated time from Hill Road to Greenlane and then recorded the corresponding actual drive-time. One estimated 35-minute trip took 26 minutes. Two estimated 19-minute trips took 15 and 17 minutes. Thirty was indicated three times resulting in one 24-minute trip, one 26-minute trip and a 28-minute trip. A predicted 23 minutes was 20 in reality while an estimated 36 minutes ended up taking 21 minutes.

Someone more mathematically gifted than myself would possibly see the suggestion of some complex pattern within such data. All I can safely say is that in my experience the estimated journey time is always longer than the actual time it takes to reach Greenlane. It's as if this electronic light board is messing with your mind. By telling you the worst-case scenario, you mentally prepare for the long-haul and then are pleasantly surprised at how swiftly you reach your destination.

Often, once you realise it's not going to be a speedy journey, you just relax and settle in for a protracted car ride. Indeed, a 2010 report entitled "Display of Travel Time on Auckland Motorways Variable Message Signs: Human Behaviour, International Practice, Policy and Customer Perception" confirms that these signs reduce driver frustration which leads to enhanced safety on the motorways. Of course, depending on how amped up and reckless you are, knowing it's going to be a slow trip can invite you to go as fast as possible while the motorway is clear in order to compensate for the certain congestion ahead. The report may say there's no evidence this occurs; my research, however, indicates that it does.

***

Three years ago a crash caused gridlock on the motorways when I was due back in the city for a birthday dinner.

Hearing this, I exited at an unfamiliar suburb with plans to head north overland. The only trouble was I didn't know where I was and my sat-nav lady just kept asking me to please make a U-turn if possible. She wanted me to get back on the motorway which I was avoiding at all costs. My journey was punctuated with dead ends, wrong turns and the occasional expletive.

Ah, Auckland motorways - can't live with 'em; can't live without 'em.

NZTA responds

NZ Transport Agency data shows that ramp signals reduce congestion and improve traffic flows. This gives motorists more reliable journey times and also reduces the number of accidents involving merging traffic.

Whenever traffic enters the motorway and changes lanes it creates a slowing pattern which impacts on vehicles further in the queue, even before the on-ramp zone. Most motorway accidents in Auckland happen during peak hours when traffic is more stop start and motorists are trying to merge together. Ramp signals reduce those stop-start conditions by providing a smoother flow of traffic.

Figures show that since the introduction of ramp signals on the Auckland motorway network there has been a marked increase in the number of cars getting onto the motorways, up to 18 per cent on some on-ramps.

It has also increased travel speeds which in turn reduce journey times. Ramp signals have also helped to reduce the peak period by up to half an hour in some areas.

The number of accidents at the merging areas of on ramps have been reduced by approximately half since the introduction of ramp signals.

Because ramp signals take a whole of network approach they generally have the biggest benefits for those making longer journeys with a slight trade- off for those taking a shorter journey.

The Transport Agency takes a whole of network approach to transport across the city and works closely with Auckland Transport to manage the entire transport system including local roads.

Travel time signals are designed to provide a guide on journey times and are not intended to be a precision tool. They are calculated by using data from thousands of sites updating several times a second. They can only tell what is happening on the network at that moment, they can't predict the future. The travel time is estimated for the stretch of road starting from the travel sign itself and not the on-ramp. By the time you have passed the sign the information may well have updated and already be showing a time that is several minutes different. The signs need to be useable and would be impossible to read if they refreshed as often as the information itself changes. The algorithm's used by the NZ Transport Agency have been audited and shown to be highly accurate. Of course estimated journey time signals are just one way for motorists to get up to date information about journey times. They can also get information before they leave home or work via our social media updates and by visiting our website www.nzta.govt.nz or by signing up to www.onthemove.govt.nz to get up to date information including free email alerts about your route.

- nzherald.co.nz