A new study commissioned by EIT Urban Mobility, TfL and the Greater London Authority has found that brake wear releases more particulate matter into the atmosphere than tyre wear and exhaust emissions combined. Studies in London, Milan and Barcelona found that overall, Non-Exhaust Emissions (NEE) make up between 68% and 88% of PM10 and up to 78% of PM2.5 generated by road traffic.
Brake wear is the largest source of NEE in cities, their emission being exacerbated by frequent stop-start driving. Significantly over 40% of these particles become airborne.
Tyre wear is the second-largest NEE, although only 1–5% of the particles become airborne, most settling in road dust, water systems, or soil. Good news for the air, less so for other areas of the environment.
Missing recommendations from report? Discourage personal road transport (of any motive power)
Build more steel wheel on steel rail transport (trams mainly).
Renew buses to further reduce emissions (Electric buses with best in class regen braking)
Research dust mitigation technology for larger road vehicles (wheel arch vacuum systems with particle traps?)
The classic electric rail vehicle solution is to use electric braking. Back in the days when electronics was expensive, simple brake resistors were used.
For road traffic I think the simplest solution would be to enforce all combustion engine vehicles to be “hybrid-ish”, i.e. having a small battery pack that can absorb brake energy, and also require all wheel drive, which automatically would mean all wheel electric braking. You’d need some additional electronics to be able to brake at really low speeds, but still just getting rid of brake particles emitted above a rather low speed would likely greatly reduce the particles.
Another solution would be to have particle filters on the air that flows through the brakes. I think that that would just not work with regular outboard brakes that almost all vehicles use, but in board brakes like some Citroën used to have back in the days could more easily be encapsulated and vented through filters.
Note that all of this also applies to rail!
Also note that this obviously has nothing to do with the biggest emission/environment threat – climate change / the greenhouse effect.
Bonus question: I wonder how much/little clutch wear particles are to emissions? In particular you reduce brake wear a lot by engine braking, but then you either need to slip the clutch a bit when downshifting, or waste fuel by revving up the engine to match the rpm of a down shift. For automatic transmissions that I think commonly uses wet clutches this might be less of an issue.
Although you may be able to scale down the battery (or supercapacitor), you still need a big heavy electric motor to do the braking. That will be why you don’t commonly see such hybrids.
Rail has the advantages of slower braking, better ability to carry weight and vehicles are typically used more frequently.
The article fails to mention that brake particulates are /probably/ less dangerous than exhaust emissions. (The comparison is difficult because, amongst other reasons, the health effects are very different.)
Or four smaller motors… Sure, to actually apply full brake power you’d need a really large motor, but to do the braking that happens in normal driving rather than the rare full brake you wouldn’t need larger motors than you need for traction, kind of sort of, I think.
I wonder if there are any data on how hybrid-ish cars brake? Also I wonder if there are any studies on how they affect particles? I would assume that they reduce brake particles a lot, and since they aren’t heavier than regular combustion engine cars they probably don’t cause that much more tire particle emissions. Well, except that the acceleration from stop is apparently great and “petrol heads” having a Prius (which itself is a contradiction, lol) tend to cause excessive tire particles due to flooring it all the time when accelerating form a stand still.
Re the danger levels of brake particles: I wouldn’t be surprised if there still are asbestos brake pads on some trains. Like heritage trains where the brake pads haven’t needed replacing since many decades or so.
That might also be a thing for “barn find” style cars.
I assume that either of these don’t cause any notable pollution overall, but it might be a good idea to impose a no asbestos rule for heritage trains in tunnels, it that isn’t already a thing. I assume that things like the heritage underground trains already are tip top renovated with modern brake shoes/pads, but for other heritage rail vehicles there might be old parts in usable condition still in use, perhaps?