I think if you are cought in one of the congestion zones without a token then you get fined. So you can by a pass that is useable for a certain length of time (or a day or long term etc etc). From what i remember they can be picked up by scanners as well as having to be visible.
Considering the congestion in London i dont know if it is working but it does meen that if the pass is only detecable visually there are enough times when you are stock still that it could be spotted if you have one or not.
It was a good race. It was dont over the London Marathon route (which crosses in and out of central London, hence the need to stop when he got inot the zone). I think technically you can buy them in advance though especially if you know you are going to be in it regularly.
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