Jarrett Walker introduces the concept of standard (i.e. rectangular) and polar transit network topologies.
Transit grids can be standard or polar, but are almost always some subtle fusion of the two. The polar grid arises when there’s a huge center on which the network logically converges, because desirable destinations are packed most tightly there.
Barcelona's bus network is a good example of a rectangular grid with horizontal and vertical routes. Utrecht, on the other hand, has a distinct polar grid focusing on it's historic centre and railway station. With this layout it's common to see shorter bus stop spacing towards the centre and/or higher frequency as several bus routes overlap.
What would be interesting to look into is if rectangular grids also show a similar pattern of shorter stop spacing or higher frequency at hotspots in the city.
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