Assume a speedster (A) isnt' subject to damaging effects of his or her speed (i.e., they can punch and hit things at their proportionate speed without hurting their hands more than they would hitting the same thing at normal speed, they don't burn up from friction, and they're not harmed by acceleration, etc. coming from their speed power), (B) can use their power to do things like run without (for instance) being limited by the ability of the surface they're running on to withstand the g-forces and friction, (C) can turn some effects on and off (i.e., run as silently as a normal person, OR interact with the air such that they produce sonic booms, throw up rooster-tails from water, etc) and (D) has all the appropriate reaction speed, etc., to be able to use it at proportionate speed (i.e., if a regular human could run through a given obstacle course, they could thread that exact same obstacle course at their running speed).
What would we see from said speedster running past, say, a double-line of people and vehicles on either side of a street at:
100MPH (do we feel a breeze?)
Mach 1+ (say 780mph)
Also, could they really pull off stunts such as making mini-tornadoes by running in circles?