Buckinghamshire CC ANPR cameras

John Wilson tugwilson at gmail.com
Mon Feb 6 11:49:14 GMT 2012

On 6 February 2012 11:10, Roland Perry <lists at internetpolicyagency.com> wrote:
> In article
> <CAJkBBXVsZSC9Wun8XCz_UqpKePe9TDrBNVnGNBOF3aacC4RiDA at mail.gmail.com>, John
> Wilson <tugwilson at gmail.com> writes
>> It would be interesting to know if the DVLA manages the numbers it
>> issues to minimise the number of collisions. The know all the current
>> registration numbers so could suppress new numbers which would have a
>> high collision rate.
> Given that the "replacements" are also the sort of characters that humans
> might confuse, reducing the number of collisions would seem to be useful, or
> are more of the collisions caused by 'weakness' of the hash, rather than two
> numberplates having the same pre-hash text after the replacement function
> has been run?

The "weakness" is caused by the substitution mechanism. Running the
test on the same dataset without substitution gives:

70% with 10 collisions or fewer
99% with 25 collisions or fewer
the highest number of collisions is 32 (2 instances)

It's possible that the DVLA to avoid issuing numbers which can be
easily confused. I might try an FoI request about this.

John Wilson

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