They wont. People who habitually break the law and/or endanger the lives of those around them take no heed whatsoever of such letters. Several forces kept records to see if it would be the case, and found no evidence that drivers who receive such a letter modify their behaviour and no link with reduced road offending rates. Hence all of those forces (including the one I worked for) no longer waste the resources doing it.
It serves only to mollify those who made the report to the police, nothing more.
I have yet to have a repeat close pass from someone I have reported who has received such a letter, and I have seen the culprits out & about whilst riding to work. In fact one Transit pickup driver I reported (usually the most obnoxious of culprits) has since overtaken me cautiously & correctly a few times now. A local Corsa driver who overtook me on a blind bend had a visit from the local officer a couple of years ago, and she overtakes me at least twice a week on my commute, safely & carefully. Obviously more effective than a letter !
This is all only anecdotal - and I have certainly been very unhappy with some fast close-passing idiots who have only had a little letter sent, rather than the full force of the law applied to their driving licences. I would love to know what the local force's criteria are for deciding whether to NIP or whether to warn.
It is certainly very disappointing & disheartening when you get buzzed too close by something like a Transit, at speed, on a clear wide road and they just send a letter (despite my saying in the report that I felt it merited prosecution).
