I think the Calendar reminders that let you know when you have to leave in order to arrive in time for an event have the potential to really make my life easier, but the travel times are all calculated as if I were driving. That's actually a relatively rare case for me; I'm far more likely to be using public transit or walking. I'd there an way to configure Calendar to use transit, walking, or biking times instead of driving times? Even better would be a way to override this setting for an individual event.
Well, I think you are talking about the Google Now notification, right?
If so, you have to change the option on the Google Now app, not on the Calendar app.
To do so, you go to
Google Now(tap the search bar on the top of your home page) then you'll see the card that estimates your arrival time to your destination, you can see like
three vertical dots, tap there and hit
Settings, now you set
Transportation Mode to
From now on, the card will notify automatically the time you need to arrive by public transportation.