I upgraded it to secondary since it is definitely a wider, busier, and certainly a higher-priority route—granted it carries a bus route, has more access to amenities like schools, a park, and a church, and intersects two primary roads plus another secondary route in the southern end. 

Now, that I think about it, I'm more on the fence. Comparing it to obviously more important secondary roads in the valley or Desert Inn or Pecos it's probably a tertiary road, but for the area it's located in, it's definitely an important road. However, it might not be too different from your run-of-the-mill grid tertiary collector road which will normally link to school and parks and at some point connect to other major secondary/primary roads in most cases, and Tree Line Drive may just be an high-traffic anomaly/exception. I'm not too sure, but I think that the way it is now probably makes sense.