How is building sidewalks a subtractive solution? Every mile of new concrete sidewalk emits about as much CO2 as driving a million miles. In some urban areas, you'll eventually make up for that. In places where people are not going to walk much anyway, probably not.

The most expedient way of "getting people out of their CO2 producing cars" is to focus on the "CO2 producing" part, i.e.

1. Get people into electric cars, which even on the current grid cuts your CO2 emissions by over half due to better efficiency, and

2. Convert the electric grid to 100% clean energy.

And it turns out there's a lot of money to be made in doing this too.