[bluGill]

Economics should define that threshold. There are things you cannot do on mass transit - get the lumber to build a new house/apartment for example. There are things you don't want on mass transit (I don't want you to take your smelly garbage to the dump via the train even though this is possible). Thus a small town will need to build roads. However as the town grows to a city eventually the minimum road to all lots is not enough. At this point we need to ask what is more cost effective: building transit in this town or building more lanes. Unfortunately transit depends on the whole system (this applies to roads too, but we started with them!), which means long term transit might have been a better answer, but right now more roads are cheaper.