1/6 is a lot lower (good!) but is that sufficient to say they're not a large source of particulate air emissions?
From my limited reading, what fraction of road dust is from tire tread is unclear. The models trying to estimate it give anywhere numbers from 4 to 48%, but may be incorrect due to the citation problem above. Experiments seem to show 4-9%, but they have trouble excluding resuspended dust.
I'd also point out that if we're worried about air quality in NYC between modes of transport, then one should look at subways since PM2.5 in stations/tubes is many times that of the street and far exceeds EPA limits.