Common sense about rules is basically two calculations:
1) What are the likely natural/ethical consequences of breaking this rule
and
2) What are the likely institutional/legal consequences of breaking this rule.
That's why you think that breaking the rule about speeding, another rule put in place to save lives, is 'common sense', just like you know that not going 110 MPH in a school zone at 8 a.m. is also common sense. So what about drug use?
When using illegal drugs:
1) Are there natural consequences of not following the rules? Heroin, yes, terrible consequences. That's what, despite the fact that my school doesn't regularly test for opiates, no one does Heroin. MJ? There's a long term cancer risk,and some people feel in impair their study habits, but most can clearly can use it long term without any serious social or physical consequences, or at least no more consequences that those who use alcohol for their recreation. Ethically its kinda neutral: its recreational, so is not exactly a civil right, but neither is it harmful.
2) Are there legal/institutional consequences? Depends on your location. Obviously anyone in a military residency would be an idiot to do drugs, since the piss test you regularly, with no notice, and have a zero tolerance policy for drug use. Medical school in Louisiana? The state considers it a misdemeanor, and the school doesn't test (at all) so what are the consequences that we're afraid of exactly?