Another reason good urbanism and walkability is super important: the emissions don't just come from the cars, they come from the excess roads themselves, too.

cement releases large amounts of CO2 when it is being produced, i.e. when the cement powder is being produced from limestone. this is due to a chemical reaction: CaCO3 (limestone) -> CaO (cement) + CO2

later, when you mix the cement with water and sand to make concrete, it re-absorbs (approx. 43% of) that CO2. you've got it backwards :D

curing reaction: CaO + CO2 -> CaCO3 (facilitated by water presence)

edit: ok i looked it up and concrete only absorbs about 43% of the CO2 that is emitted during cement production. Source