The 20 minutes are wall clock, not game time. And it translates to roughly 1.66 years of game time, or 1 year and 7 months.
Assuming that fast forward is double game speed, it would be 10 minutes wall clock, and double fast forward would mean 5 minutes wall clock. The real values might be longer because the in-game time would count, and this time is delayed by the necessary calculations. Any frame rate drop or stutter will affect in-game time somehow.
On the other hand, it won’t matter. Whenever a person is given a new destination, because new buildings are raised in other cities, the currently effective travel time will be calculated. This is based on the obvious values for walking distances, and the approximated times for waiting and traveling with bus or train. These times are affected by service frequency: Have more trains will service a stop more often will reduce waiting time.
If the destination can be reached within 20 minutes, the connection will be made, and the person starts traveling this route. However, if due to traffic jam, the actual travel takes more than 20 minutes, this will not abort the journey – it will only affect the future choices of how to travel. If public transport used to be fast enough, but then got worse, maybe the car is fast enough. If even the car is too slow, the destination will be replaced with one that is closer.
For cargo this means that the demand for a route will plummet, and trains will travel almost empty. This usually is devastating for your economic success, because driving demand down is easily done within a month – driving it up is limited to one unit per month increase, and the stops in increase because the factory has to be upgraded.