My experience with no-smartphone Garmin traffic reporting
In early 2022 as I write this, I now have decent experience with the best non-smartphone traffic reporting for Garmin GPSes. I have used both the regular RDS traffic reporting using the TA20 and GTM35 (which is the same as the GTM36), and the HD reporting using the GTM60.
When using an older Garmin nuvi or DriveSmart model in the United States, traffic coverage only exists in major metro areas. The RDS coverage map is here and the HD map here. At the time I write this, I happen to live in an area that has full RDS and HD coverage.
The Garmin models I've used with traffic support: nuvi 265T (RDS w/GTM35), nuvi 2250 (RDS w/GTM35), nuvi 2455LMT (HD w/GTM60), nuvi 2555LMT (HD w/GTM60), nuvi 2457LMT (HD w/GTM60), nuvi 56LMT (RDS w/TA20), nuvi 2599LMTHD (HD w/GTM60), DriveSmart 50LMTHD (HD w/GTM60), Drive 52 ("fallback" RDS w/GTM60).
Does it even work?
Yes. The RDS-TMC signal at best starts receiving in a few seconds, and at worst takes 2 minutes to receive if I'm parked in a place where the signal is blocked by something (garage, carport, etc.)
Ordinarily, I'm either using the Drive 52 or nuvi 2599, either of which can be set to automatically reroute around traffic during navigation (as in the "Optimize Route" option is set to "Automatic") when it knows about it. I have both set this way, and yes, it works.
Is it accurate?
This literally depends on where I'm traveling.
On major highways and interstates where traffic coverage exists, yes, accurate. Major highways and interstates are patrolled routinely and have cameras in many places, hence why the traffic reporting is so good on those roads.
For everywhere else, traffic reporting is usually slower or nonexistent. I have encountered traffic instances (mainly due to road construction) where it wasn't reported at all to the RDS-TMC system, and therefore never appeared in the Garmin. I have also encountered traffic reports the Garmin said were there but were already cleared when I passed through. Neither happens due to a fault of the Garmin but rather those who are supposed to make timely traffic reports and didn't.
Is it annoying to use?
No. On most Garmin models you can change the frequency of how often you are notified of traffic reports. You also have the option of just shutting off traffic reporting at any time when you don't need it (but you do have to remember to turn it back on).
Is the traffic coverage decent?
As noted above, traffic reporting coverage is only for major metro areas, which is arguably the only place where I would ever need it.
Is traffic reporting even necessary?
In all honesty, no, but it's still better to have it than not.
Where the traffic reporting works best for me is in unfamiliar areas or areas I infrequently drive in. The last thing I want to worry about whenever going somewhere unfamiliar is how to get around traffic on roads I don't know.
RDS or HD traffic reporting on a Garmin does work, and I hope it continues to work for a good long while.