You are probably hoping for a magnitude, but all I can provide is the lowest Modified Mercalli Intensity which mentions being felt by people driving cars - MMI VII "Very Strong." So that would correspond to being in the meizoseismal (area of maximum destructive energy) area of a pretty powerful quake. That same VII includes "difficult to stand," and "considerable damage in poorly built or badly designed structures."

I'm not a professional, but my guess would be near the epicenter of a magnitude 6+ quake would be necessary, though I'm sure there is anecdotal evidence for much smaller quakes being felt by drivers.

MW
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