JohnW wrote: ... Of course TC - they're motorists themselves, aren't they?
FWIW, I don't think that's really part of it, although it's true that the way the system has adapted to the growth in motor traffic has not been good.
The CPS was formed to improve the standard of prosecutions - to stop miscarriages of justice - and little thought was given to what would happen to summary cases, especially traffic offences. There was a lot of streamlining, including extending fixed penalties, but contested case is back to full criminal justice mode. The system was creaking - on the point of collapse - prior to the formation of the CPS. In the early 1980's, as an operational inspector with a bit of prosecuting experience, I was regularly working 60 - 80 hours overtime in every four weeks helping out with my learned friend act.
The system has always depended on most people pleading guilty or, more recently, not contesting a ticket. One problem with tickets is that they only work well in clear-cut situations. Anything with either a lot of twiddly bits (like most traffic offences) or anything with a grey area can be difficult. eg the ticket issuer might be unfamiliar with all the twidly bits, or may be challenged over their opinion if the offence is not black/white. There's not the same supervisory checking as the traditional report for summons and so on.
To keep this from getting even longer, I'll mention that in two prosecutions of cyclists defended by the Cyclists' Defence Fund - one about traffic lights and advanced stop lines, the other about a pavement cycle track - the CPS would not stand and fight. That wasn't anything to do with being cyclists themselves.
PS As I've posted umpteen times, AFAIK, the battle against footway parking was lost when the courts decided the bill poster with two wheels up on the pavement was OK. Ging into the newsagent's for a packet of fags and a couple of scratchcards isn't in the same league as somebody with a bucket of paste, big brush, roll of posters and a set of ladders but the principle quickly spread