> So fentanyl is the drug most people overdose on, or is it killing more people than heart diseases, traffic accidents, etc do?

The numbers for other types of fatalities are available so you could do a quick comparison:

42,000 motor vehicle fatalities in the US for 2021 (all ages), so way above that

Heart disease is more unclear, I mostly found age-adjusted risks/etc. It looks like at best I could find, around ~400,000 americans in that age group have heart attacks (eg. MI) each year, though the death rate is obviously going to be lower than that.

So, it's definitely possible. IIRC "death by misadventure" and the like are super high on the list of causes of death for young people (18-25), so very possible fentanyl is way up there if not at the top.