In message , at 12:02:10 on Mon, 5 Nov
2018, Someone Somewhere remarked:

Arguably it should be e.g. £5 per journey, £10 for the most polluting
vehicles (and maybe an even higher figure for e.g. certain lorries),
and £2 for those that are emission free at the tailpipe (as they are
not entirely polluting free in general and there still needs to be an
aspect of congestion charging). A journey could be classed as passing
through the congestion charge boundary inbound (with an exception that
twice within a very short time was obviously due to a circuitous journey).

Putting aside the policy issue of charging at all for a moment, that
doesn't work on a topological basis. One car could be driving all day
long while staying inside the emissions zone (remember we are talking
about the N/S circular very soon), whereas another which just happened
to 'live' near the boundary could do half a dozen short trips spread
throughout the day, but nevertheless crossing the boundary.

Unless you set your "very short time" at say 12hrs, which isn't at all
what you meant.
--
Roland Perry