This is based on what I do and have done all my working life. If it takes 30 minutes to get somewhere give myself an hour. Like I say you can never be to early
My dad always drummed into me about (a) never getting a second chance to create a good first impression and (b) catch the train before.
If I'm driving to work I'll leave earlier than I have to go account for traffic conditions. If I get the bus I'll try and get an earlier one, I just see it as common sense.
I've also lived by this ethos. For me it was my Mum that drove it into me. Leave early, plan for any problems, allow for things to go wrong, have an alternative etc. My drive to work can take 20 minutes on a good day. On a bad day it takes nearly an hour. Trains, planes and automobiles never run on time. It would be remiss of me not to allow for that. I also firmly believe in personal responsibility. If I know the train is regularly late and I needed to be somewhere and I didn't plan for that. It would be MY Fault.
*Disclaimer : I work for a TOC. Apparently that informs my opinion and I'm not allowed to every be a 'passenger' or a 'human' or an 'adult'