Garmin

Varia RTL515 Rear View Radar with Taillight

I think it's important to understand what this product does, exactly. It tells you, very reliably, when something is approaching you from behind (i.e. moving faster than you, getting closer to you) and flashes the light differently when that is happening, regardless of the mode. I've used this with both a Wahoo Bolt and a Garmin Edge 830. The Garmin can control the light modes from the unit (and changes modes between day and night), the Wahoo only displays the radar overlay (which shows up to 4 different vehicles and how far behind you they are). This is a fantastic product and I highly recommend it. Once you're moving over 20 mph, the wind makes it very hard to hear an approaching car, and I'm much more relaxed on long, paved descents with this on my bike. There are a few things it will do that may not be desired, but I consider them forgivable overall. 1. It will detect bikes approaching you. This is only a problem if you're riding with people who you might pull ahead of and then they move to catch up frequently. 2. Sharply curved roads may result in repeated detections for the same car as it enters and leaves the radar's field of view 3. In cities and other heavily trafficked areas, you might start to tune it out because the alerts will be quite frequent 4. If a car is following you and matches your speed very closely, the radar will lose it. You still need to be aware of your surroundings. Note that with both head units you can temporarily disable the audio alerts, which I will do sometimes if I'm getting too much of things I consider to be "false positives". Otherwise the battery life is great, the mounting system is ok, and it's a little mysterious why such a sophisticated light doesn't recall the last used flash mode (you have to cycle through them each time you turn it on, unless a Garmin is controlling it).