Years ago Heathrow Airport allowed frequent travelers, resident and non-resident, to volunteer to have their retinas scanned, and enter the country through a separate "Iris" lane. It clearly worked very well, and it reduced the volume of travellers in other entry queues as well. For no reason that I can understand, they scrapped it, after years of success, and went to a system with three more layers of complexity, as if an "eye" is easy to fake! To this day, why a fingerprint and retina aren't enough to identify you, if you volunteer to "sacrifice" your privacy (not much of a sacrifice when you can be virtually strip searched on a whim), is mystifying. The technology has existed, the will clearly doesn't.