Doug asked: "I'm sure there is/was a reason for the design and production of lightweight tricycles...what is/was it? I've seen the new "pedicabs" in San Diego, but the Longstaff etc doesn't seem to be designed for a commercial use."
I suppose they were originally developed because they can be ridden without needing to balance yourself. At slow speeds this is indeed the case, but to ride a trike at high speeds is quite a bit more complex than riding a single. Just ride behind a trike and you will understand what I mean. As far as the reason that they are still around today is because as Mick correctly pointed out, they are an absolute hoot to ride.
I started riding trikes 25 years ago during the snow-filled winters in Canada's capital, Ottawa. As the list-members from Ottawa can attest, the winters are not terribly friendly and the road conditions can change incredibly much in a short time. The trike was therefore a way to know that I could always get home. You can ride whether there is black ice, slush, snow or clean roads. We even held races on the Rideau Canal and Dow's Lake, which any Ottawan will gladly tell you are part of the longest skating rink in the world.
Today I ride it because it is fun and riding one well gives me greater satisfaction than riding a single.
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Steven Maasland
Moorestown, NJ