This is one of the big differences between an air cooled Harley and all the modern water cooled products. Air cooling in the world of Harley requires large amounts of air to be constantly moving over the engine, and they don't provide fans to keep that air flowing at low speeds or when sitting in traffic. They also don't provide enough surface area to allow more efficient radiational cooling at those low air velocities, mostly because they would then run into overcooling in normal riding or cool weather. The situation could be greatly improved with the simple addition of temperature controlled fans and higher capacity oil coolers, but for whatever reason Harley seems to be loathe to admit there even is a problem, much less step up to address it. But that doesn't mean you can't address it yourself; the aftermarket does offer some solutions like better oil coolers (Jagg) and some also offer fan kits that can be added to the coolers. Of course, there are also the parade fan kits that mount where the horn currently resides to blow air between the cylinders.
I avoid all of that by trying to avoid having to stop, and when I do get caught in a traffic jam or by a train, I shut the engine down and don't let it sit idling. Better to just pull off and sit than to inch along for miles at a time, IMHO. Saves a lot of wear and tear on the clutch too.
Jerry