Commuters tend to be creatures of habit, and I am no exception. My return journey timing is a bit more variable, but I tend to leave for work at about the same time everyday. The same is true of many other people I pass along the way, so they become a kind of fuzzy human clock.
You know where you normally tend to pass other commuters travelling on your route in the opposite direction, so if you see them earlier along your route, it's a pretty safe bet that you're a bit late. Similarly, if you see them when you're nearer to your destination, you might be a bit earlier than usual. I recently realised that I was subconsciously using these kinds of encounters to roughly gauge my timekeeping, and found it quite funny that -- inconstant creatures though we are -- much of our behaviour is predictable enough to set your watch by. Things do fall apart a bit if your 'reference person' happens to be running early or late too, though, but I have a watch to check.