As far as roads go, there are no real rules - roads can be built on every tile...

I have a mod plan (http://forums.civfanatics.com/showthread.php?t=181037),
in which one key feature is a new set of rules for the so called "imperial roads":

To build imperial road the following rules must be met:

1. On any 2x2 tiles max. 2 tiles can have imperial road,
2. On any 3x3 tiles max. 3 tiles can have imperial road,
except when (2/b.) 2 tiles with road within the 3x3 matrix are either horizontally or vertically aligned - then 4 roads are alligned
3. Imperial road can only be build to join an existing imperial road (starting from a city)

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The question for this thread:

Does anybody (coders, mod-makers) know the logic, rules, routines
the AI uses when deciding where to build roads?

It would be useful in order to think about
how road building rules in the game might be changed...

Interesting!
I'm afraid I can't help you. But keep in mind that when two imperial roads cross, you would have 5 roaded squares in a 3x3 tile square.

and at least 3 in one 2*2

They won't cross! ;)

But they can join...