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The problem with 3 driveways per block is you've just made a really low density set of stores. This means that you massively increase the average distance someone has to travel to get to the store they want. One of the main reasons to separate streets and roads is that it encourages tightly packed small shops which (especially when combined with mixed zoning) create low average travel distances. Lower travel distances mean fewer cars on the road (cause more stuff is in walking/biking distance) and therefore lower traffic.