For me, there are two main things need to be taken into account when discussing the legalisation of substances. How addictive is it? (If it's highly addictive then you are no longer making a choice) and how extreme are the side effects if taken in large doses (which is likely to happen if it's addictive). How damaging is it to the body? Cigarettes for example are addictive, but you could smoke 40 in one day and not OD. Now I've never taken heroin, cocaine, meth etc, have no desire to, but if you took several hits of that in one night, how likely are you to OD?
Could one stupid decision or a couple of stupid decisions potentially end a life? A couple of nights getting pissed on alcohol isn't likely to kill you (from side effects of the drug itself, not from external effects such as driving or falling in water)
You can't compare drug taking to driving a car, that's illogical. There are safety measures in place to minimise the dangers of driving through engineering, education, training, testing, restrictions, etc etc. putting a substance into your body is a roll dice.