I meant to post an update with my initial radar experience which addresses this question from
@kahn ... I followed Ben J's advice (thanks
@Ben J ) and installed RideWithGPS on my Android phone. It's quite good in its own right. Map, position and some ride stats all displayed, or just the ride stats (with limitations - see below). Integration with the Garmin was a doddle as there's a menu option for it. The radar data is then displayed overlaid on the left (or right - another option) side of the screen. It all works very well and I was convinced after just a few miles. Multiple approaching cars - even on the narrow roads we have here - were indicated correctly. The audible beep is useful and the 'all clear' sound is very welcome. I've only done about 20 miles, but there were no missed cars (and no false positives, which are less of an issue) and I quickly learned to stop looking over my shoulder and started to trust the technology.
The app has some limitations - no cadence data, no battery etc - some of which might be fixable with a subscription, a more intelligent user or more technology - but I just reprogrammed the top tube display on the SL to show the relevant data and, as they say, "Job's a good 'un". I'm unlikely to subscribe, purchase more ANT+ sensors or get smart(er).
RWGPS even syncs with Strava at the end of a ride. However, the features I use/need are only a subset (I don't care about segments, split times, king of the mountains or leaderboards ... I rarely feature on any of them) and so I'll probably simply switch from Strava to RWGPS. Route planning in RWGPS seems competent but I've yet to try the turn by turn ride assist.
Two final comments. I used a QuadLock handlebar mount for the phone which is a bit expensive, but feels very secure and puts the phone where I want it (over the stem, but connected to the bars). I also replaced the Garmin seatpost attachment (rubber bands, no thanks!) with
one of these.