Ideally keeping the speed up* to avoid rad kicking on - not much can do about that if in stop n go traffic, other than careful route planning regarding time to avoid the congested periods
* Double whammy - idle or low speed produces less current and of course low road speed means no air flowing through radiator and fan kicking on.
selecting a lower gear will help the former (assuming it IS actually moving at all) but of course not much can do about the latter.