As mentioned, Rome was a prior inhabited location, and grew organically. It also had a lot of hills that impeded regularity of streets.
There are some parts of it that are more "regularly planned" than others (particularly after the 67 fire).
But if you look at other Roman Colonies and Cities, you will see a very regular plan.
Take Ostia for instance. Even though it has some irregular streets around the periphery (the West being from previous establishment and the S and E being from new growth), it is laid out on a Regular Castrum Plan.
If you look at the Cities of Winchester, Cheshire, Lancaster, and any other city in England with the suffix "Chester" you will find a core of regularly laid out streets. The same is true of London's core, where the old Roman Castrum was laid out.
Pompeii also had a certain regularity to it, although less so that colonies established ex nihilo (from nothing), such as Timgad in what was Roman Libya (now Algeria). They had a very regular plan for all of their growth.
So
With looking at the cities and towns of Gondor, certain questions need to be asked:
• Were the cities built where there was a prior established village or town before the Númenóreans arrived?
• What was the geography like?
• Did the Númenóreans build with Concrete, as the Romans did?
• Since there seems to be a preference for circular layouts among the Númenórean cities of Gondor (and maybe Arnor, we don't know), it is possible that the Númenóreans had a regular plan for their military camps, which was used as a model for their cities, and was this "camp" radial in nature, rather than square, like the Romans (a radial plan would be easier to construct with the proper knowledge and tools, both of which the Romans Lacked – and a radial plan would be more defensible)?
• How much of an influence did the Númenóreans have over the locals where they settled. Was their influence syncretic or dominating, or domineering?
These questions, when given certain answers, will guide how the Númenóreans in exile in Gondor and Arnor built their cities and towns (which, BTW, I am convinced there were more "Cities" and "Towns" in Eriador, Gondor, and Rhovanion than were listed by Tolkien in his works given many other statements he makes about his world).
Remember, Tolkien said he was Discovering the world.
This means that technically, he had not yet discovered all there was to know about the world, and had relayed the info he did have through the eyes of vary Pastoral Hobbits, who would not have known to ask certain anthropological (even though this is not quite the term to think of things relative to Middle-earth) questions about the societies they were visiting.
They probably didn't think to ask "How big is that town?" Or "Are there other cities in the Western lands of Gondor that rival any in the Eastern parts?"
Or, "Even though Osgiliath is technically "abandoned," are there any people who have moved into the still intact buildings left there?"
And so on