Addendum: I casually placed the tag within the windscreen shaded area (I wonder if people know what that's for) and thought that the job was RTFM, THE TAG NEEDS TO BE REGISTERED BEFORE USE. Anyhow, job dome. More Sanef information here
Cars 2m or under are class 1 and can use the faster height restricted lane, which can often be approached at 30kph; cars with a roof box are class 2 and must use the other 't' lanes,but are still charged at the class 1 rate. Motorcycles,class 5, cannot use the left height restricted lane and instructions say that and Libet't tag on a motorcycle must be handed to an operator at a manned booth. Note the tag is NOT cheaper than cash/card use,it is a convenience feature only and there are annual 6E charges plus a month of use 5E plus a handling charge for currency conversion.