Set up a crane shot. The crane base should be on tracks. Set the distance your character needs to walk and shoot that at normal speed.
Take the same distance and shoot people passing by (on paths that do not intersect the main characters path) moving the crane really slow.
Comp the two shots together - speed up the one where people move by to match the speed of the shot where the main character walks. Cut a mask around the main character. You may have to mess around with masking parts of the image out or in, depending on the pavement and how solid and shake free your camera rig/crane is.
One other thing to consider is to lay down a chroma fabric and have the main character walk on it so that you can pull a key for him/her.