The Varia's radar sees cars and the cars see the Varia's light. Use this excellent combination radar and tail light to increase your safety (and enjoyment!) while cycling.
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In short, this is one of the few bike products I can entirely endorse as long as you fall into one of the large categories of riding situations (see the Who and Where section below). I often find that funny, as most that I describe it to (especially experienced cyclists) look at it as a novelty. But, anyone that's ridden with one for more than a ride or two won't give it up.
Its radar (one of the two main features) seems magical. In short, it detects cars approaching from behind and sends a signal to a compatible Bluetooth/Ant+ device , which then displays it for you to see. So, if you're using a Garmin or Wahoo GPS, or a phone with compatible app, you can see when cars are approaching without needing to glance back. This is both safer in most cases and massively increases confidence for cyclists new to riding next to traffic.
A device that is attached to the rear of the bike. It serves two key purposes:
- A rear tail light, with adjustable settings (various flashing rates, can change when traffic approaches).
- A radar that can detect and send information on traffic approaching from the rear to another device (see the Using with Another Device section for more).
Who and Where
Most riders can benefit from a Varia, but I've found them most useful for:
- Those riding on roads that aren't high traffic. If traffic always running, the Varia will always show someone coming, making the radar less useful. In that case, a standard rear taillight (without radar) would be less expensive. While I get some value inside city limits, I find this most useful for riding outside the city where traffic is lighter and I want to know when someone is coming up.
- Newer riders that may be nervous in traffic (when the novice rider becomes more experienced, he won't want to give it up!).
- Gravel riding.
In opposite of the above, the Varia will be slightly less useful for:
- Those that only ride on bike paths.
- Mountain bikers that don't ride near roads.
- Experienced riders that prefer more traditional ways of managing traffic awareness.
Safety. Fun. Enjoy using technology.
Why not? Cost. Another thing to manage/charge (if you want to use another taillight, you'll have to manage the charge on that, too, although less frequently). It would also take some creative attachment if you use a rear rack bag that sits over the tire, a large saddle bag, etc. (in those cases you'd have to attach it to one of those items, rather than the seatpost).
I use mine on every road or gravel ride. The only time I leave it at home is if I'm mountain biking or 100% certain I'll only be on bike paths.
Using with Another Device
As noted above, to be useful, the Garmin Varia needs another device to display the traffic information captured by the radar. Fortunately, Garmin created it with open standards and any device with Bluetooth and/or Ant+ support  that has implemented support for it is compatible.
Some specific features and how to use the Varia will vary for each supporting device, so make sure to review the additional pages in the links to the right for more (for example, each brand shows oncoming traffic with different icons, some devices allow quick overriding of taillight blink patterns, etc.)
Currently-compatible devices include:
- Garmin Edge series (1000, 820, 520, and 130 and higher)
- Wahoo bike GPS devices
- Smartphones (iOS and Android) with supporting apps (Garmin's Varia app and RideWithGPS are the most notable, although others may add support over time)
- Select Gamrin watches (Forerunner® 935, fēnix® 5 series, and vívoactive® 3)
 Only the Garmin Varia 515 has Bluetooth support, which is what a Smartphone will need to work. The older Varia 510 model will still work with Garmin and Wahoo devices over Ant+, but does not have Bluetooth support.