that's a strawman. and also not very much. A lot of places either have no sidewalk or were already designed with a side walk and street in mind. so given that side walks usually don't even have enough space for a full lane of traffic, it would have 0 impact on traffic.
Personally, I think subways should be the backbone of every city transportation system. But surprisingly even very environmentally conscious cities choose to forgo subway systems.
Yes, that's a strawman. Subways, roads and no sidewalk at all.
And your answer is 0... How? Bridges and tunnels between buildings? People evolving to fly?
Every person would require door to door taxi — tremendous increase in traffic. Same with bike lanes, take them away from Amsterdam and city would choke. Every Amsterdam driver benefits from less cars on the road.
Personally, I think subways should be the backbone of every city transportation system. But surprisingly even very environmentally conscious cities choose to forgo subway systems.