My commute is 9.5 km (6 mi) one way, with 17-18 traffic lights on my way. Best I've done is around 24 kph average (15 mph), and I don't see much options to improve on it, since no matter how hard you pedal between the traffic lights you always have to stop at one or the next. The only way would be doing it sometime in the night and skipping red lights.