Good point. For me the issue is the variance which governs the time that I have to leave. For the subway, if I give myself a 10 minute margin beyond the median time, I'm quite likely to be on time, for walking or biking if I give myself a 5 minute margin, I'll almost certainly be on time. For driving on the other hand... as per the article, a trip with a median travel time of 17 minutes can take me over an hour when traffic backs up.