Why not cesty instead of ulice?
Cesta - road, way. Ulice- street.
So ulíce is both singular and plural?
Yes, that's right. In the nominative. :-) Just notice that the "i" is short - ulice.
Why not nove ulice and just ulice nove?
The word nový (nové) is a part of the predicate (jsou nové) in the sentence (Jsou ty ulice nové?), while in your suggestion (nové ulice) it develops the noun ulice.
I do believe the ty is optional.
You have to use it, because the sentence refers to particular streets and saying that those are new (ty ulice - the/those streets).
It may not be optional but the sentence without "ty" is accepted.