As with most things in life, common sense must prevail. The environmental conditions should always be taken in to account. If the area has been dry and the risk of starting a fire in the surrounding vegetation is high, of course you should be extremely cautious in using any flame-generating device. However, the use of reflective triangles would likely be beneficial only at night, when headlights are turned on, so you might still have to resort to another type of warning device. I can tell you that even the use of multiple strobes on an emergency vehicle is not always enough with many of the drivers out there on the roads now a days.
One experience I had with warning “devices” down in Ecuador: We travailing a paved road, so our speed was moderate (we were in 65’ VW Bug- so the speed was not too fast), suddenly we came upon a huge bonfire off to the side of the road. The driver looked at me and kind of shrugged his shoulders as to indicate he had no clue as to the meaning or purpose of the fire. We learned very quickly that the fire was a warning that the road was missing up ahead, as we plowed into a 3 foot sand hill propelling the VW Bug several feet into the air. As we landed, we looked at each other; half laughing hoping there would be little damage to the VW Bug, as we were miles from anywhere. Fortunately, no major damage occurred and we were on our merry way, just with more caution. Pete