The 20 minute rule is not to break your head around it. I mention just to make clear that you need to make fast connections between places, otherwise people won’t use your services. That is an easy way to explain why people prefer cars instead of buses, trains etc, depending on the route.
Measure the time will be very annoying and hard, considering the roads have different speed limits.
Its just a tip, and with that, make fast connections between the cities. For sure most people will use it.