Hi I just came across this and need to measure back and assume this 20m starts as soon as the passenger wheel crosses the outer bus lane line .I've seen this 20 metre "rule" quoted a few times, but it is not in any regulations, and just an indication of what could be an appeal based on de minimis. No adjudicator is bound by any 20 metre rule, as there isn't one.
"Given that the most common complaint among drivers is that they moved to the left slightly too early in order to turn left, the most significant ground for appealing a bus lane fine is that drivers are permitted 20 metres of grace – approximately five car lengths. So if the supplied photographic and (in some cases) video evidence does not show the car travelling more than 20 metres in the bus lane, there are good grounds for an adjudicator to throw out the PCN."
Anybody used this and won.