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.)
Yeah, the story setup makes it seem like your mission is actually urgent, so I also only long rest when it’s absolutely necessary.