It's interesting to see how the term "junk miles" has been perceived on this thread. Junk miles (in a training/coaching perspective) refers to miles that have no specific training value, meaning they do not create physiological adaptation.

For example, one of my clients commutes to work in SF each morning and again in the evening. Her commute is very short (3 miles each way) and is interspersed with stop lights. She doesn't get a sustained effort and her HR stays low. There is no true training adaptation that will take place during these 3 miles, so those are referred to as junk miles.

Another example is a traditional training route from SF into Marin county. In order to get out to the main loop, you have to ride through a bunch of little towns (again, stop & go like the first example). Even though there is time on the bike, the effort is not sustained and never reaches a high enough intensity to create adaptation, so those would also be considered junk miles.

I don't think there's any judgement intended in the term. It's simply a term that's trickled down from the coaching world to describe time in the saddle that doesn't really have training value.

Here's another example that might be meaningful to some of you. Let's say you workout at the gym twice a week. Your total time commitment to train includes the time to transport yourself to the gym, change clothes, and put your belongings in a locker (let's say that's 30 minutes total). Now, even though all those things are necessary to do your workout, none of them help your body adapt (ie there is no training stress). So, those 30 minutes wouldn't be considered training time (ie they would be junk miles).