So, eyes down, you keep walking. But when to look up? Too soon and the smile and acknowledgement will take place but it will be followed by the awkward ten steps as you edge towards a comfortable communication distance. Leave it too late however, and it looks like you're simply trying to ignore them. When did greeting people we know get so complicated? Perhaps staring them out is a solution...would certainly be an ice breaker and a good conversation starter. And better to err on the side of creepy than rude. Sometimes.
Once the comfortable, socially acceptable distance at which acknowledging someone is established the second obstacle arises. What to actually do. Stop, kiss and hug with open arms, enquiring about their health and giving an answer in great detail resulting in a two hour coffee shop conversation? Or smile, say 'hi' and continue walking, even if still exchanging pleasantries? The issue stems from everyone approaching the situation in a different manner with different expectations, based upon their preferences, time availability and mood. There is no easy answer, indeed it is doubtful there even is an answer.
Maybe life would be easier if we all walked around with masks on?