believe or not, there are formulas for this, you need to know how much of the explosive is there and how much, if any, fragmentation will form. these distances can easily go into kilometers
An explosives safety separation distance, ESSD, from a substance, article, or structure with reacting material, specifically burning material, is one where an individual would not receive second degree burns and would not be exposed to hazardous debris (<79 Joules) at a density greater than one fragment per six hundred square feet
The table below from this site shows an appropriate evacuation distance...
I'd presume the amount of explosive munitions in that burning truck was on the high end...