Maybe, but it's an interesting concept.

My city put bridges over a lot of the major roads for people to cross. A few roads have a tunnels underneath as well.

I imagine you don't even have to put too many of them in, before any pedestrians give up and drive---having tired of needing to climb and descend four flights of stairs or, worse, walk an extra block along ramps, just to cross the street.

If I'm in a car, I'd like to not brake-accelerate-brake-accelerate but rather move in a constant tempo. If I'm a pedestrian, I'd like a car-less safe environment to walk.

Make the whole city 20mph max? Fine by me :)