1. You're completely right. There is a gantry right above the SPUI junction. However, by convention, the `traffic signals` node should be placed at the location where vehicles need to stop. In this case, it's before the stop marking line visible in Bing Maps Aerial. That's where I placed the traffic signal, and that is why I made that segment a motorway. I've added the gantry to the database and changed it back to secondary.
2. Thanks for sharing the info; I had no idea I had to do that first. There's already a discussion in the OSM Forum about highway classifications, so I'll likely change my tags to make more sense. Once again, thanks!