As a pedestrian, I would never step off the pavement unless I
could see that the road was clear: I would never *make* a car
stop for me with the single exception of a zebra crossing.

That's the observed behaviour of most pedestrians in the UK,
which drivers have come to take advantage of. It requires
agility and a plan B to assert your rights. And we don't have
presumed liability as a backup.

I would say that the one place where pedestrians should NEVER cross
unless the road is clear is at a junction, because drivers cannot
see round the corner until they get very close,

Transponder receivers are so cheap they should be mandatory in every
vehicle.

The transponders are even cheaper and could be worn by cyclists,
pedestrians and motorcyclists who want to stay alive.

The problem with transponders is that it is only possible to know that
something of interest is nearby.

With several frequencies each set for a distance, 15m, 30m and 45m. you'd have a good idea of how nearby.

It is impossible to know whether an
absence of signal means there is nothing of interest or something of
interest is not communicating.

How is that different than visible bike lights at night?

When 10's or 100's of these things are in range

You should be slowing down if there are a lot of pedestrians and cyclists around the corner or curve.

how would a human sort
them out?

After all, you only want to run over a select few!

Once you have your "cheap" transponders and receiver, the
information needs to be processed and the important results presented to
the human in an easy to digest visual, not audible, fashion.

92 kV electrodes in the drivers seat should do the trick.

Probably a
HUD with markers trying to direct the eyes to look at the actual target.

Wouldn't it be better PR for motorists if you quit calling pedestrians and cyclists "targets?"

Is it still cheap?

Nothing is cheaper, not even neon paint.