by planning your route, you can save time and fuel. i drive two different routes to and from work. the trip home is a longer one, but it saves me 15 min of wasteful idling in traffic.
by memorizing traffic signal durations, i rarely stop at all in the early mornings to work. sometimes conventional thinking is not the correct course of action. for example, an upcoming stale red light may dictate acceleration, and inversely, a stale green light may require slowing down to maintain momentum(it may turn red and then green again before a complete stop is necessary).
To add to that, if you've got a GPS, bring it along for the ride. Typically theres a setting to show you the FASTEST or SHORTEST route. One is based on time, the other is on distance.
I've used both, and often times the SHORTEST route doesn't give me the BEST route, but it can knock 25km off my route in some situation. Whether or not that 25km worth of fuel ends up being chewed up because I'm being told to go through a residential or commercial area with lots of stop lights, versus the 25km of highway travel, I can't be 100% certain.