I got passed doing 14-15 on my commute by a guy with a high end bike in full kit, then saw him turn in to a business about 1/2 way along my route so sometimes it's about length of commute too.
I average 14.5 on my route with VERY few stops (along a MUP with overpasses, underpasses, etc) and my route is fairly flat. I feel like that is hustling for me and I find it difficult to increase my average speed on my current SS equipment. Funny thing is my speed hasn't changed since I went from a 24 speed MTB to a single speed track bike for commuting!