Tiny particles from the wear of vehicle brakes, tyres and roads already make up about half of the air pollution particles from traffic. This will be getting worse because our car fleet is getting heavier and there are no clear policies to control this pollution source.
The issue known as “car bloat” and “autobesity” refers to vehicles bought today being bigger than the ones they replace. This affects parking and raises concerns about safety for other road users. A US study showed that children were eight times more likely to die when struck by an SUV compared with a normal passenger car. Bigger vehicles also have a greater impact on the climate and air pollution.
The trend towards heavier cars began in the 1990s. Over the last decade the average European car weight increased by about 9%. Sales of small SUVs increased by five times and large SUVs sales increased by seven times.
Several toxicological studies point to potential damage to health from breathing brake, road and tyre pollution, but there are still many important knowledge gaps, not least around brake wear and particles from electric vehicles.
Dr William Hicks and his team, from Imperial College London, have created a computer simulator to study air pollution from brakes. In his latest study, six different vehicles undertook simulated drives around London and an international standard vehicle test.
There were big differences between vehicle types. The brakes in the large SUV had to work twice as hard as those in the small compact car, resulting in greater particle pollution.
The batteries in electric vehicles typically make them 20% heavier than their petrol and diesel equivalents. However, hybrid and battery vehicles have regenerative braking: they use their electric motors to slow down, putting this energy into their batteries whenever they can. They only use their friction brakes during hard and slow-speed braking.
Despite their additional weight, the brakes in the battery and hybrid vehicles worked less than their petrol and diesel equivalents. Brake pollution was reduced by 88% in the international driving test and 68% in the London test.
Hicks said: “I was also surprised by the extent to which driving styles can influence brake-wear emissions. Aggressive driving and braking, which is common in city driving, causes more direct abrasion between disc and pad, but also heats the brakes up, causing more ultrafine particle emissions too.”
“In the long term, smart traffic-control and driver feedback should encourage efficient driving. But in the short term, individual drivers can reduce air pollution by driving smoothly. This could be encouraged through traffic calming measures (20 mph speed limits should help), media campaigns and driver awareness courses.”
The impacts of vehicle weight are not confined to cars. An earlier study in London found links between increased numbers of heavy lorries and increases in the types of particle pollution that come from brake, tyre and road wear.
Despite this, technology solutions are still in their infancy. These include devices to collect tyre particles right next to the wheels, and sensors to ensure that wheels are aligned correctly. Paris and other French cities are looking towards increased parking charges for SUVs.
Prof Roy Harrison, from the University of Brimingham, who was not involved in the computer simulation study, said: “The results give new insights into the influence of vehicle weight and driving style upon regenerative braking in electric and hybrid vehicles, and the associated particle emissions reductions.
“They show that regenerative braking can lead to substantial benefits, but should not be used to detract from the effort to develop engineering solutions to this important emission source.”
