Black cabs have only this year started using 2.8L Euro6 diesels, so, pretty much all of them are running older tech. Their own website says the Euro6 is 71% more efficient than Euro5. All old cabs, plus the 13-year-old million-mile 53-plate buses I pedal behind up Dog Kennel Hill, spewing out visible particulates, should be addressed ASAP. Lead by example.