I've got the BMJ sat in the kitchen so I'll have to look at the methodology, however I used to do the same commute as the author and the time difference was huge mainly depending on traffic etc. Anything up to 10-20 mins extra if the traffic was shit.
** cough **very proud moment....( I have the pleasure of working with him)and, he is very fit... so will make a bigger difference if you are a bit chubby [like me] fatkid.. the large sample size satisfied the BMJ proof readers. The stats hold up.