This can be done quickly unless you get far into a game on any map where it may get harder due to the greater amount of traffic (200-1000 trips).
As time passes, locations build up pins, until they become one giant pin.
Basically, once a big pin forms, create a traffic flow that is greater than the build up, causing the pin to become small pins again. The easiest way to do this I have found is with a motorway or by destroying a road to build up a pin intentionally, then repairing it.