The (logical) fallacy is in the word "must". Let's look at the statement again:
> if an aspect of a system benefits a group, that the group must have planned said system aspect
This is incorrect because sometimes groups benefit from things they haven't planned. If the statement were changed to "that there is a good chance the group planned said system aspect", it would no longer be a fallacy.
It's the same as the slippery slope fallacy. Slippery slopes are real things. Sometimes A does get out of hand and cause B. Saying that if A happens, B must happen is where it becomes a fallacy. Changing this to "If A happens, there's a good chance it will snowball and B will happen" also removes this fallacy.
That said, there is precedent for traffic laws being set by fuel consumption concerns and there was industry pushback against it that led to its repeal. https://en.wikipedia.org/wiki/National_Maximum_Speed_Law
The difference between Drive 55 and the stop signs is that Drive 55 was enacted by a single legislative act, and stop signs proliferate due to a combination of state code, municipal code, civil engineering practices and limited imagination. Traffic circles are currently proliferating just as easy, and reduce fuel consumption.
Stop signs’ utility in shutting up squeaky wheels or appeasing anti-car agitators are a simpler and more direct explanation.
It's a constant thread amongst all conspiracy theorists: GW Bush BENEFITED from 9/11, therefore he must have had some hand in planning it. Tech monopolise BENEFITED from COVID, therefore it's a "plandemic". Fuel sellers BENEFIT from stop signs, therefore they coordinate putting more of them up.