In my experience, the Puffin crossings are setup correctly precisely 0% of the time.
If you are going to pay presumably a lot more money for all of the extra detectors and electronics then they need to deliver 2 things as mentioned by OP: 1) They make sure that anyone on the crossing has time to cross rather than stopping traffic for a fixed amount of time (useful outside schools) and 2) If there aren't any people crossing, the traffic should be stopped for a short amount of time no worse than if they were just a normal Pelican crossing.
However.
Even when no-one is crossing or in some case someone crossed and is about 50 metres up the road, the crossings are still usually on red for a total of often 20 seconds, which is way longer than most Pelican crossings that are on red for usually 5 to 10 seconds max.
I don't know if no-one notices or cares but it is really annoying!
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