Petrol price at the pump does factor into it. For longer distances, I look at public transport and weigh travel time and ticket prices against fuel costs.

For shorter distances, great bicycling infrastructure helps a lot. Buses are nice but cycling is often as fast, with the benefit of being completely door to door and leaving exactly when you want to. But this benefit only works if people feel completely safe on the road, which means mostly seperate cycle paths, and removing cars from the busy city centers. Not having cars there makes shopping more attractive, because it's now easier to walk from store to the store across the road.

That same infrastructure then also helps make trains more attractive, because you can set up cheap bike hire at stations. This then means your journey can be bike-train-bike door to door, rather than walk-bus-train-bus-walk (or worse).

It's a complicated connected mesh of things.