Transport planners use terms such as latent demand and induced traffic to explain this, but it can be explained in plain language.
If a city's population is growing, a road will become busier. This continues until the amount of traffic at rush hour can't grow any more. The congestion stops any more people from using the road.
In other words, a congested road puts people off using it. So, if the Auckland Harbour Bridge is congested in the morning, people are more likely to catch the bus to work instead of driving across the bridge. If they were thinking of going shopping in the CBD, they might decide to go somewhere local instead to avoid the traffic. Or, if the trip wasn't that important, they might just stay home.
The flipside is that if we make a road less congested, more people will drive on it. So if a road is expanded from two to four lanes, traffic speeds will increase at first, but as more and more cars use the road, congestion will grow again. The end result is a four-lane road with the same congestion and speeds as the original two-lane road.
If all we care about is how fast the cars are going, we're no better off. We're worse off. Because on the four-lane road, there are twice as many people stuck in traffic. That means twice as much time lost.
This reminds us that we need to think less about roads and cars, and more about getting people to where they want to go.
In Auckland, we've been building more and bigger roads for years, but at peak hours our roads are still clogged. If we remember that bigger roads encourage more cars, this isn't surprising at all.
If we start thinking about people, instead of roads and cars, the alternative becomes obvious. Our goal shouldn't be free-flowing car traffic, because we know in the long-term it will never happen. Our goal should be free-flowing people.
One way to achieve this is building rapid public transport. This needs its own protected space, like trains, or buses on a busway.
Rapid public transport is a great answer to congestion, because the congestion proves there are a lot of people trying to go in the same direction, and this is exactly what public transport needs.
Another way to get free-flowing people is better infrastructure for walking and cycling. For example, routes through parks and greenways help people walk and cycle away from congested roads.
Maybe the best way of all is to design our neighbourhoods and cities better. The more things people can do locally, instead of having to travel across town, the less time they will spend stuck in traffic. Road building undercuts local businesses and services, because it encourages people to drive across town to go shopping instead. The opposite is intensification, which brings more people into a town centre to live in high-density housing and apartments, and attracts more local businesses and services.
That's why neighbourhoods and cities that want to be more liveable are making roads smaller. This frees space for busways, cycleways or new public areas, it pushes people out of their cars or it encourages them to do things locally instead of travelling across town. The result is fewer people stuck in traffic, healthier local businesses and neighbourhoods that are much better places to live.
This is a timely reminder for the Auckland Council as it considers whether to reduce spending on big new roading projects. Liveable cities don't try to make traffic go faster. They free people from traffic.
Dr Jamie Hosking is a senior lecturer and health and transport researcher at the University of Auckland.
