One of Auckland's busiest intersections explained

Thursday, 1 April 2021

The story of the intersection at Te Irirangi Dr, Ti Rakau Dr, and Botany Rd in East Auckland.

There is a lot going on where Te Irirangi Drive, Ti Rakau Drive and Botany Rd converge. These are important arterial routes in East Auckland, packed full of commuters. These roads butt heads at one of the country’s biggest shopping malls, the Botany Town Centre. There is a Countdown, a Pak ’n Save, a Warehouse, a New World. There are a lot of other big shops. There are a lot of shoppers.

About 30 lanes of traffic collide at this particular intersection.

It is extremely busy during weekday rush hours, although the concept of rush hour is not really a thing any more – peak traffic now extends for up to three hours in the mornings and afternoons. The road is also beset by cars at weekends, particularly between 12.30pm and 1.30pm.

If you frequently drive north through this intersection, it is likely you have experienced long delays. If you commute through this intersection from the city centre, you have probably lined up a good selection of podcasts.

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About 5500 vehicles travel through this intersection every hour during peak time, says Miguel Menezes, the network optimisation lead at Auckland Transport.

There is one more thing about this intersection: it has recently been used as a testing ground for a traffic experiment that Menezes and his team hope can make Auckland flow a little better.

The intersection of Ti Rakau Drive, Te Irirangi Drive and Botany Rd in East Auckland is one of the busiest in the country.

At first glance, the science of traffic is complicated, brimming with intricate mathematical equations, and words such as ‘’flow’’ and ‘’density’’. All of this appears to explain the obvious (when there are too many vehicles on a road, there will be congestion).

But these fundamentals allow transport engineers to get traffic flowing as efficiently as possible, for example, identifying precisely how many cars should be allowed on to a main road from a motorway on-ramp.

Allow too many cars on and the whole thing clogs up; allow too few and the road is not performing at its capacity.

So how does traffic work? What type of quirks are there? And why is this particularly Auckland junction being used as a test case to get traffic moving faster?

The basics of congestion

Imagine you are driving the only car on the road. You never have to brake suddenly, there are no sudden traffic jams (we will come back to those later), there is no need to merge, there is nothing to slow you down.

Now, imagine a handful of other cars arrive at this road. You might not even notice it but the complex mathematics of traffic suggest the addition of a few more vehicles will slow you down a touch.

The road is big enough to handle these new arrivals and the traffic flow (the total number of vehicles passing a given point in a given time) increases.

Two lanes of Ti Rakau Drive, highlighted, can carry thousands of cars every hour.

Now imagine more cars join in. It is getting busier. Traffic is still flowing nicely though. Again, more cars join. Now things are beginning to slow down.

As the speed of all these cars drops off, so does the traffic flow. More cars join. It is now clogged up. This is congestion. This is bad.

But before that point was reached, there was a sweet spot where this imaginary road hit what is called its capacity. This was when the flow was at its maximum, when the number of vehicles and the speed of those vehicles was just right.

On a single lane of road in a city, with no impediments such as stop or yield signs, the capacity is about 1800 cars every hour. On a motorway, it goes up to about 2000 cars per hour.

Two lanes at our Auckland intersection are highlighted below. If those two lanes were never impeded by red lights, the capacity of both would be about 3600 vehicles per hour.

But as soon as they cross another two lanes of traffic, their capacity is pretty much cut in half (assuming both have green lights for the same amount of time).

So a kilometre to the east of this point on Ti Rakau Drive, where there are no roads to cross or stop signs, it may well be that 3600 cars are zipping past every hour. But once they hit the intersection, there is an immediate bottleneck.

That is just the start of it. This Auckland junction is really complicated. There are cars turning right. There are pedestrians. And once you factor this in, the capacity of the road drops off even further.

We've condensed 15 minutes of activity at one of Auckland's busiest intersections into a single minute.

So what to do?

There is this rule in Auckland called the Traffic Signal Times Protocol. Essentially it says the maximum cycle-time at an intersection or junction should be 120 seconds.

Junctions are programmed to run through a cycle, where each of the lights turn green. Think of it this way: if you were to drive up Ti Rakau Drive and hit the red light just as you reached the junction, you would be waiting for a little while before the lights turned green again. Just how long you wait is the cycle-time.

Auckland Transport wants to move as many people through traffic lights as fast as it can. It wants to minimise delay for all users.

(It focuses on people, not vehicles, which means sometimes traffic lights are skewed to favour buses.)

It uses a system called the Sydney Co-ordinated Adaptive Traffic System (SCATS), which collects traffic data and subsequently picks the best timing for green lights.

At nearly every set of traffic lights in the city, there are sensors near the stop line. The purpose is two-fold, Menezes explains. The first is to detect a vehicle, the second is to measure the distance between the cars as they pass through.

Say there is a queue of cars and the light goes green. The first car drives through, if the second car passes within two seconds, the system assumes there is a queue and the light stays green.

“When all those cars are passing together, the system says: there must be a vehicle, there must be a vehicle, there must be a vehicle, and it extends to a certain maximum time,” Menezes says.

The problem is the maximum time may not be ideal. The SCATS system makes assumptions using mathematical modelling but there are limitations. It does not (and cannot) understand human driving behaviour as well as engineers can.

“If we leave SCATS uncalibrated, it will keep giving long greens,” Menezes says.

Long greens may seem great. You may well have sat in a long queue of traffic, cursing wildly at nothing, as the lights ahead turn green for what seems like mere fractions of a second and the car ahead tiptoes forward. But it turns out long greens are not always the best bet.

Squeezing the most out of it

What causes those mystery traffic jams?

The rule of thumb is that for every two seconds that a traffic light is green, one car gets through. That rule can help determine how long lights should be green. You would think long greens = more cars get through. Job done.

It is not that simple: “All you need is for one person at a red light to be distracted for one second and that (two-second) modelling breaks down,” Menezes says.

People get distracted all the time. They check their phones, they check their make-up, they daydream. They throw the best computer modelling off.

(Above see how the roads “flares” out from two lanes to four to get as many vehicles through as possible. This is called getting the benefit of the “stop-line” capacity.)

In real life, there is also something called the accordion or concertina effect. A blog post on blog.ferrovial.com explains this really nicely.

“If, for example, there are 2000 cars stopped in a single-file line, when the first one starts to move, the line of vehicles stretches out like an accordion. The second car starts a second later, the third two seconds later, and so on to the last one, which starts moving about half an hour later.”

The gaps that emerge down the row of cars create inefficiency. The cars at the front are bunched together. The cars further back are spread out.

The concertina effect also explains phantom or mystery traffic jams, when someone suddenly brakes or slows down, sending a wave through following traffic that causes everyone to slow to a crawl.

Late last year the cycle-time at the Te Irirangi Drive, Ti Rakau Drive and Botany Rd was, on average, 140 seconds.

The changes have made the intersection more efficient.

As the mathematical model does not really pick up on the concertina effect, Auckland Transport (AT) ran a series of trials testing cycle-times of 120 seconds, 110 seconds and 100 seconds.

They found that by reducing the cycle-time, good things happened. At 120 seconds, about 200 extra cars were able to pass through the intersection during the peak hour. Travel times were reduced across the board during the 100-second trial. And the length of queues dropped with shorter cycle times.

The key takeaway was it is OK to reduce cycle-times. It makes things better. In the end, they decided a 100-second cycle time during the afternoon peak was the most efficient way to minimise the delay for most users.

With this approach, Menezes says, “whenever there is a green, from whichever approach, a tighter parcel of vehicles passes through”.

Extend that over an hour, for example, and more vehicles are getting through green lights.

Menezes says AT is now considering testing these shorter cycle times across the city. He says not only do shorter cycle-times mean a more efficient traffic flow, they mean pedestrians have less time to wait before crossing a road.

Going beyond one intersection

In his University of Canterbury office, Dr Mehdi Keyvan-Ekbatani, a senior lecturer in transportation engineering, uses two cups of rice and a funnel to illustrate how traffic flow works.

First, he pours the first cup of rice smoothly into the funnel. It passes through comfortably, gathering in a bowl underneath. Then he dumps the entire second cup in. It clogs, next to nothing getting through.

It is not a new concept but it is timeless – an excellent illustration of how traffic flow works. Pour cars into a system at just the right rate and the flow is constant. Allow too many in and you get congestion.

There are 19 “gated” routes into Christchurch’s centre (surrounded by the four avenues) in the simulation.

Keyvan-Ekbatani uses this to explain how traffic flow is managed when, for example, a motorway on-ramp merges with a busy main road.

The key is, he says, to ensure that the main road is at the sweet spot – where the traffic flow is maximised.

The sweet spot is precarious though. Too many cars will push the main road into a state of congestion – so the key is to ensure only a handful are allowed to enter at any one time.

That is one road. But Keyvan-Ekbatani’s work is focused on thinking about the system as a whole.

The model above shows how a single road behaves. What Keyvan-Ekbatani has done is to extrapolate that out across a load of roads.

You can’t think of a single road on its own, you need to think of it as part of a system. If you cut your arm, it affects the rest of the body, he says.

He outlines a concept he has tested in Christchurch, called “gating” or “perimeter control”. The idea here is to think of Christchurch’s centre as a single system.

Each of the red lines illustrates a ‘’gated link’’ where the traffic flow is contained to get the city centre running as efficiently as possible. Essentially the “right amount” of cars are allowed into the city centre at any one time, to ensuring it is operating at capacity.

The simulation, he says, has yielded some extremely positive results, with little or no congestion. He and his team also saw a significant reduction in bus delays and in total emissions (congestion is bad for the environment). Gating also allows traffic managers to prioritise buses via specialised bus lanes or signals.

But doesn’t this just generate more congestion outside the city centre?

Keyvan-Ekbatani says it is possible to manage entrances into the gated area in an equitable way. So for instance, if there are few cars at one gate, you give more green-light time to others.

Why should I care about how traffic works?

Well, Auckland traffic jams are estimated to cost our economy nearly $1.25 billion every year.

There is also a clear environmental impact. About 21 per cent of all greenhouse gas emissions are caused by transportation. And about 70 per cent of those emissions are from cars, SUVs, utes, vans and light trucks.

A car’s fuel consumption and emissions are also heavily influenced by speed. One study, for example, found traffic congestion led to an 80 per cent increase in fuel consumption.

Traffic congestion is not an easy problem to solve. There is no silver bullet. Roads need to be managed properly, driver behaviour can get better, public transport can take cars off the road. Automated cars, for example, could make the concertina effect a thing of the past.

The problem is: about 1.7 million people live in Auckland right now. Another million people are expected to move there over the next 30 years.

About 25-30 per cent of Auckland’s main arterial routes experience congestion during peak times and the level of congestion is only going up.