That only works if you have symptoms. Surely you can't be unaware that many (maybe even, most) people with the virus have no symptoms and will therefore be unaware that they pose an infection risk.
If you're not coughing or sneezing, I think it's reasonable to suggest that the mass of material being emitted from your respiratory system is much lower, and hence there is a correspondingly much lower risk of transmission anyway.
If you are coughing and sneezing, you shouldn't be on public transport, and we should be strengthening the measures to make sure that people aren't adversely effected if the have to self isolate. Let's not forget that they're not even entitled to priority delivery from supermarkets at the moment; despite their sacrifice being one of the most valuable tools we have.