As mentioned above, for several reasons it is desirable to aim for a simple route structure with a minimum number of routes. In particular, given likely resource considerations, the more routes that are operated the lower the bus frequency on any one route; international research clearly demonstrates that “choice” riders (those with a car or other alternative available) are unlikely to use low-frequency routes.
In any event, route spacing is in many respects dictated by the form of the urban fabric. Typically in most cities, bus routes run along the main roads bounding blocks. This to an extent dictates route spacing, and can make it difficult if not impossible to run parallel routes less than (say) 1 km apart.