This is an important point, and one you can see in practice in downtown Boston walking above the I-93 tunnel - lots of land is still used for off-ramps and on-r

This is an important point, and one you can see in practice in downtown Boston walking above the I-93 tunnel - lots of land is still used for off-ramps and on-ramps, which tends to have high-speed traffic, whose presence is dangerous and discourages walking, which is the lifeblood of the city.