Possibly Clear Up what is meant by the "Traffic Jam" reports
Instead of having "Moderate", "Heavy" and "Standstill", why not clarify these categories a little more. They could possibly be instead "Congestion", "Slow Down" and "Standstill". I think these adequately reflect the different degrees of traffic jams that occur. Congestion is still dangerous but might not reflect a great decrease in speed. When drivers will have to decrease their speed that is useful to know, and of course "Standstills" are self explanatory.
An alternative option could be to elaborate on the provided categories more instead of changing them. They just seem a little ambiguous.
I do agree they're a bit ambiguous, but I think using descriptive words like congestion and slowdown would be worse. For example, you seem to think congestion is minor traffic and slowdown is in the middle, but I would think slowdown would be minor (like in a construction zone) and congestion would be slow-moving, more in-line with "heavy." I think this may be a case of people just using their best judgement and hopefully Waze is smart enough to determine the most commonly reported type and use that.