Me in BG3 when the enemies are perfectly grouped around a single innocent NPC. Doing that calculus on how much that NPC really needs to survive the encounter.
The wording of the feature is that you can cause a number of creatures you can see to auto save on the throw and take no damage if they would have taken half.
That will just turn the same dead commoners in that r=20' sphere extra crispy. I don't think there are any spells in 5e that increase their AoE when upcast and not the damage, duration, or number of targets.
(Either way, if the street is significantly narrower than 20' then a lightning bolt is going to maximize the carnage.)