Unless you are fortunate enough to have a route that makes sense to time trial on, I think you'd be better off finding a good time trial route somewhere and testing yourself there once a week when you're not commuting. Trying to do your commute as fast as possible can lead to poor decision making in heavy traffic. If you are dealing with stoplights and traffic, it's also not a very good test of fitness, as there are too many other things that influence your time.