Turning car off at stoplights

[rz]

I’m in the middle of a 5 week business trip in Japan. Something I’ve noticed is that many drivers turn their cars off as they approach a stoplight, then start them back up when the light turns green. I even noticed a bus do this the last time I was here, but have been on several busses and none of them has done it on this trip. I think I saw a sign that said people should do this out of courtesy for people along the side of the road so there are less fumes, but this seems counterintuitive to me. Wouldn’t restarting the car generate more fumes? And wouldn’t it waste more gas? Gas is expensive here, about the equivalent of $6/gallon.