Living in Europe I would imagine that in theory, and probably in practice, I could hit the road with a car and drive all the way to e.g. Malaysia, because the road network consists of "millions" of roads connected to each other so that I could eventually hit my destination by just driving. Other land masses besides Afro-Eurasia have their own mainland-wide road networks which likely connect the distinct points of the continent. The same goes with islands such as New Zealand and Australia.
I'm interested in where I can find the largest road network that shares a landmass with, but is disconnected from, a larger ('main') road network?
In this example there is a primary road network (in black) in a land and an additional road network (in green) which isn't connected to the land's primary road network.
To define how roads are connected, roads connected by a bridge or by unpaved road count as a connection, but e.g. ferry connections do not count (as one can't drive continuously from one end to other - however if the ferry takes you to an island, the road network would be part of another land mass's road network).
The US's gaps in Interstate Highways seem like a case where the roads are still connected to each other, so they likely do not provide a sufficient answer. I have read of cities that have been built in the "middle of nowhere" which could have independent road networks. Also vast land masses with a challenging nature such as Antarctica or Greenland possibly have separate road networks which are not connected to each other nor to any other land mass. Maybe one of these could be the answer to the question.