Some things to consider when determining the road and street layout of a city are:

1) The geography of the site. Is it a flat valley floor or is it hilly? Flat terrain is more likely (but not necessarily) to have a grid layout. Rich people usually live in the hills, and the roads are curvy there.
2) The history of the city. Is it an old city? It's more likely to have curvy, narrow streets. Newer cities would have been planned with cars and freeways in mind and therefore would have a network for people to drive in and out of suburbs.
3) Are there important or historic buildings there? A cathedral? A stadium? A crowded downtown area? A shopping district? Those are going to have major roads leading to them.
4) Is there a main thoroughfare like 5th Ave. in New York or Market Street in San Francisco? Major roads are going to provide access by way of linking directly or indirectly to it.