I would think it is saving gas in N

N is virtually equivalent to idle. You'll probably agree that idle is the least fuel consumption state, along with load increase, presumably other conditions are the same, the fuel consumption is increasing...

In practicality, if green light waiting time is counting in seconds, it doesn't make too much difference. If it's longer than 5 minutes or longer due to accidents, putting gear in N, or better yet, turn off engine, is the way to go...