Canadian cities from Edmonton to Montreal are lowering speed limits, primarily in an attempt to save lives. But slowing down may also be an easy way to reduce greenhouse gas emissions and air pollution — not just on urban roadways but also on highways (and even the high seas).

According to Natural Resources Canada, driving a vehicle with an internal combustion engine at 120 km/h burns 20 per cent more fuel than driving the same distance at 100 km/h. An Ontario law that requires trucks to install technology to limit their speed to 105 km/h was estimated to have reduced greenhouse gas emissions by 4.6 megatonnes between 2009 and 2020.

That's largely because air resistance increases exponentially at higher speeds, reducing a vehicle's fuel efficiency and generating more pollution per kilometre. 

In addition, certain pollutants such as nitrogen oxides are generated mainly at higher speeds, said Marianne Hatzopoulou, professor and Canada research chair in transportation and air quality at the University of Toronto. 

That's the reason the Netherlands recently cut its daytime highway speed limits from 130 km/h to 100 km/h. But Hatzopoulou said cutting speed limits can reduce emissions on city streets, too.

In both cases, it's not just your maximum speed that counts but how often and how much you speed up and slow down, as a result of things like congestion and traffic control. "All these acceleration events will actually lead to higher emissions," she said.

If a speed limit is set at 70 km/h, for example, cars try to accelerate to that speed at a green light, cruise very briefly at 70 km/h, then rapidly decelerate at the next red light. If the speed limit is 40 km/h, there's a lot less acceleration and deceleration, Hatzopolou said.

Lower speed limits have indirect climate benefits, too. They can discourage car travel and, by making streets safer, encourage walking and cycling. 

The city of Prince George, B.C., advocates reducing downtown speed limits to 30 km/h specifically to encourage walking and cycling as part of its 2020 climate mitigation plan.

"Slower speeds … actually create liveability," said Sandy James, a Vancouver-based urban planner who has been advocating for lower speed limits for years. 

James thinks it's a message governments are starting to get. She noted that Europe is beginning to mandate the use of intelligent speed assistance in new vehicles. The technology limits a vehicle's speed based on local speed limits. 

Meanwhile, the U.S. National Association of City Transportation Officials recently recommended setting lower urban speed limits based on safety, as well as local urban density and activity, rather than the traditional method of gauging how fast drivers are comfortable driving.

Regardless of the posted speed limit, individual drivers can actually reduce their fuel consumption (and emissions) by 15 to 20 per cent through improved driving habits alone, according to Ralph Sims at Massey University in New Zealand.

Here are some eco-friendly driving tips from Hatzopoulo:

• Stick to the posted speed limit (or go slower if there's congestion or traffic signals ahead).
• Keep a steady pace (cruise control can help).
• Accelerate and decelerate slowly.
• Limit the number of times you change speed — this may mean fewer lane changes and less passing.