I see some discussion of zero speeds on this forum, but no mention of zero distances. With some great help on the #help-solve forum, I was able to handle those. But I think the instructions should mention that a trip of zero distance should be assigned according to the rules, but does not affect the availability of the taxi to which it is assigned. That taxi becomes immediately available again.(I was making the mistake of not assigning these zero-distance trips at all, figuring you don't need a taxi to go nowhere:-)