It's easy to separate motor vehicles from bicycles and pedestrians --
just use those spikes like in gated communities. *Cyclists can slip
between the spikes or stop and step over them as can pedestrians. *A
motor vehicle will get 4 damaged tires.

Separating cyclists from pedestrians is trickier. *The first obvious
solution would be to install parallel bars like on cattle stops on
sidewalks/pavements. *A good wheelman, however, will just hit these at
an angle like he does oblique railroad tracks. *Maybe some combination
of patches of ice and grates would work but that would be expensive to
install and maintain and dangerous to pedestrians as well.

Moreover the weight of a cyclist + bicycle averages about that of a
pedestrian,

What?

What distinction are you drawing between a cyclist and a pedestrian?

More to the point, why?

both an order of magnitude less massive than a motor
vehicle. *Even more compelling a motor vehicle often represents 2
orders of magnitude more kinetic energy.

As a practical matter, is there really any reason not to reclassify
cyclists as pedestrians as far as cycling on the sidewalk/pavement is
concerned?

Yes.

A bicycle is a vehicle, not a pair of feet.

Some kids' shoes have small wheels in the heels.

True, but what point is it you are struggling to make?
A person riding a cycle is a cyclist (at that point)

When does a pair of wheels become a bicycle?

Did I mention bicycle?

If you want to digress from the OP feel free to start another thread.

When a drive chain is added?

It's wouldn't be hard to snarl up the judiciary as well as
uk.rec.cycling with a lot of inbetween designs.

Gun nuts use the classification/camels nose approach all the time with
great success. *Some guy in Florida realized he would have to pay a
bigger fee to register his floating dock as a dock than if he
registered it as a boat so he registered it as a boat.

My father once got an article in _Reader's Digest_ about a Navy man
who couldn't get the Navy to pay for moving his canoe across the
country. *He filled it with dirt and flowers, called it a "planter"
and the Navy was ok with that.