Half of all commuters switch to cars on rainy days, but how we react creates ongoing ripple effects which add up to an even bigger slow down, traffic engineer Doug Wilson says.
In the past 24 hours, many areas have been been hit by heavy rain and storms as a weather system described as an ‘atmospheric river’ crossed the country, with Tasman, Auckland and Northland bearing the brunt.
Even without stormy weather, staring at the bumper of the car in front of you while waiting to get to work or back home is a daily reality for many of us – but add some rain to the mix, and the commute can quickly turn back up into a snail slog.
Or does it just feel that way?
Apparently not. For a start, up to 50% more people switch to cars to commute when it rains, University of Auckland Associate Professor in transportation engineering Doug Wilson told Checkpoint.
People who might normally walk, cycle or use public transport switch to cars when it rains, because it becomes more convenient, he said. And that added volume of vehicles acts as a spanner thrown into the works.
“Our transport systems and our user behaviours are dependent upon what we would call ideal conditions – the capacity reduces as we move more away from those ideal conditions,” Wilson said.
“… Adverse weather is a good example of adverse conditions that affects capacity of our transport systems. So human behaviour changes, people get worried about the weather, so they take longer following distances, they might brake more carefully than if it’s in dry conditions – so that affects capacity and also safety.”
Each person’s behaviour in response to risk is different, which creates a mix of different driving patterns on the roads.
“The more variance you get between the drivers – which is humans who are actually keeping that system in equilibrium – the greater the difficulties it creates to capacity of the system during those times of variance,” he said.
“Some drivers will be more cautious than others, some will follow much more closely. It doesn’t matter what the weather is like, others think … ‘I need to be more careful here’.”
Wilson said there were bad drivers everywhere around the world, but speed was a particular problem in New Zealand drivers.
Bad behaviours were also dynamic and changeable in different situations.
“Like if we’ve had a long dry period, the first wet period that we have we forget some of those behaviours that we learn over winter for example, so at the end of a summer, often our behaviours are worse for that first wet period,” Wilson said.
“We are not robots, we are not consistent. All of us make errors in our driving. Most of those don’t end up being an incident or an accident actually occuring, but obviously those behaviours are difficult to manage given that we’re not robots.”
Pinch points could occur on the roads in places where some people see more risks, and drivers slowing down can potentially cause a ripple effect further down through the traffic flow and form queues that are slow to resolve.
“You might commonly see that when you get people rubbernecking with an incident occurring, then that creates an effect on the other side of the motorway because people are looking and taking their view off what they should be, which is driving and keeping consistent. So it creates a problem on both sides of the highway, even if it’s not a problem on that side,” Wilson said.
Even after adverse weather conditions clear, ‘shock waves’ in traffic patterns could still remain for an hour or two afterward, he said.
But weather was just one of many variables contributing to traffic patterns. While experts could use modelling and algorithms to try to predict traffic flow, those mostly calculate for ideal situations, whereas real life patterns were a “very dynamic situation”.
While autonomous self-driving cars did not offer an immediate solution to remove complexity by taking out the human variance factor, Wilson said. That is because any shift toward the new technology means a transition period, with a mix of human drivers and automated vehicles together sharing the roads – creating even more variation between different driving behaviours.
rnz.co.nz
