I just looked up, using my city's transit website and published timetables, how long it would take me to get to my wife's workplace from mine. I walk every day during lunch, so I have a pretty good idea of how long it takes me to get to the nearest stop (since I walk past it on my usual route). I got 45 minutes. Google transit came up with 43. Maybe I'm a slow walker.

Whenever I take public transit, they're been pretty much dead on. You might have noticed some issues, but I haven't.

As to the driving, I'm pretty sure they use location tracking of Google users who have opted in to tracking to determine the time it takes to go between a given pair of waypoints. If it's underestimating your time, you might just be a slower than average driver. I have a 90 minute drive I take every couple of weekends with my wife. She always pulls up directions, since it will warn us about accidents or other slowdowns on the way, and we're always within five minutes of the estimated time of arrival. For what it's worth the roads are 65 and 55 mph on the way, and I do 74 or 64 mph.