Climate explained: does building and expanding motorways really reduce congestion and emissions?
In the latest Climate Explained article for The Conversation, UC’s Professor Simon Kingham considers the real impact of building and expanding motorways when it comes to the reduction of congestion and emissions.
Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.
Fuel efficiency is optimised for driving at around 80kmh and it decreases the faster you go above that. But with speed limits up to 110kmh, people are likely to drive above 80kmh on motorways — and this means building and expanding motorways will actually increase emissions.
Many countries, especially in Europe, are now looking to lower speed limits partly to reduce emissions.
In addition to speeding, rapid acceleration and braking can lower mileage by 15-30% at highway speeds and 10-40% in stop-and-go traffic. If building or expanding motorways did reduce congestion, the smoother driving would be a benefit.
But this assumption is not backed by evidence. Research shows even on roads with no impediments drivers brake and accelerate unnecessarily, increasing congestion and emissions.
One of the arguments for future autonomous vehicles is that such braking and accelerating should not occur and emissions should reduce.
New roads, new drivers
The most significant impact new and expanded motorways have on congestion and emissions is the effect on the distance people travel.
Historically, engineers assumed cars (and more pertinently their drivers) would behave like water. In other words, if you had too much traffic for the road space provided, you would build a new road or expand an existing one and cars would spread themselves across the increased road space.
Unfortunately, this is not what happens. New road capacity attracts new drivers. In the short term, people who had previously been discouraged from using congested roads start to use them.