Is that actual "causing havoc" or BritishDriverSpeak's version of "causing havoc"?
Well, if you get two parked on the double yellows the bin lorry or the tyre delivery lorry can't get passed. They don't need to stop there, just get by. However, by the time they get to the point where they can see that bit it's a bit late. It's a one-way street (so they can't come the other way) and they have a queue of cars behind them, which then all have to reverse out onto a main road about 20yards from it's junction with the high street. There's another one-way street parrallel but going the other way and traffic lights at the big junction. They can gridlock the centre of town for some considerable time.
Conversly, there are areas with double yellows where cars can be left happily all day, causing no obstruction to anyone but accruing parking tickets. Seems a little odd to decided whether a car is an obstruction or not based on who's driving rather than where it's parked.
Didn't know about the toll exemption, learn something new every day.