I've been pondering on this topic myself and searching the web for a while, but haven't come up with a definite solution. Here is the situation in short:
I'm working on a site, which routes point A to point B. There are literally thousands of such points. There is a browsing functionality included on the site, but since there are nearly countless number of combinations of A and B, I will be limiting As and Bs only to the more popular ones.
Here come the two questions:
1. The easy one: How many combinations of A and B are practical to include in the browsing functionality as to not 'overwhelm' users and/or search engines? If, say, I take only 10 most popular points from each region I still end up with well over a million pages, which is probably beyond practical. Or not? Does it matter at all how many pages will there be?
2. The tough one: how do I structure links for browsing? Naturally I'm not going to just layout a million links divided in however many pages - that would be impractical for all - human user and search engines. So, for a human user I figure the browsing logic will probably make sense: select origin region -> select country of origin -> select municipality -> select point of origin THEN: -> select destination region -> select destination country -> select destination municipality -> select destination point. However, I have serious doubts that search engines are going to want to dig so deep, considering there is very little valuable information (besides lists of countires, municipalities, etc.) until you finally get to the resulting page - this is where the interesting stuff is supposed to be. So, the question is - what linking structure will be practical for human users AND search engines (and if there is such a way, how to tell search engines not to bother too much indexing interim pages and just continue to the last page).
Sorry if this sounds confusing, I'm ready to provide examples and further explanations if necessary. Would be very interested to hear comments and suggestions.