The episode: “Last Clear Chance,” the short attached to “Radar Secret Service,” ep. 520
The riff: The riff is interjected by Crow during a lecture from a friendly highway cop on staying awake and alert while driving. The cop says “You should stop at least once every hour. Get out, stretch your legs…”, at which point Crow mumbles “…take some bennies.”
The explanation: As it turns out, “bennies” are a slang term for Benzedrine, an early form of amphetamine that was being abused as a stimulant as early as the late 1920s. It would appear that the drug was used in a manner similar to the classic “trucker stay-awake pills,” either ingested or inhaled for a pick-me-up. Urban Dictionary agrees with this assessment.
Novelty factor: I assumed the explanation was along these lines thanks to the context in the episode, but I’ve never heard the term before.