As weed causes drowsiness, it can cause you to fall asleep at the wheel. It causes problems with depth perception which can lead you to stop too soon or not soon enough or turn wrong, which could cause an accident. Also, if a person goes into a full body stone I've seen people do this even after there has been awhile after they smoked pot they will pretty much lose the ability to drive. It can also cause attention problems people who are high on pot are often easily distracted, which could take their focus from the task at hand.
And let's not get started on what could happen if the drug is laced with something, which can be quite common. This comes from my own personal experience with the drug mainly how I've seen other people react when high, but also from the few times I smoked pot when I was younger I personally think that it should be illegal to drive while heavily influences by any substance, which includes alcohol, marijuana, cocaine, hash, heroin, whatever.
Unfortunatly it is much easier to detect alcohol on someone's breath via the use of a breathalyzer than it is to detect the presence of drugs. Generally drugs need a blood or urine test. You can see symptoms of drug use for example red eyes but many visible signs of drug use can be caused by other things red eyes could be caused by allergies It would be very easy for a person to refuse a blood test, and hard to get a conviction for impared driving when a person refuses to take the test.
With alcohol, there is generally the smell of alcohol. This could also happen with pot, but you could very well be the designated driver after a party where pot was consumed and smell of pot.
Additionally, many other drugs do not give off an odor like cocaine or IV drugs. With alcohol refusing a breathalizer is considered an admission of guilt.
It would not be the same with a drug test as the drug can stay in your system for a period of time, while not affecting your ability to drive, and submitting to a drug test that would show you have consumed drugs at some point could have an impact on your job or your reputation. In this case you would not be guilty of impared driving, but refused to be tested for other reasons. The only reason to refuse a breathalyzer is if you think it's an infringment on your rights or if you are actually guilty.
As for the dangers and addictiveness of pot vs alcohol and cigarettes, there is a great deal of conflicting research on this. Pot can cause cancer when smoked, just as cigarettes can, however people don't generally smoke pot as much as they smoke cigarettes. Pot smokers generally smoke once or twice a day or less usually less.
Cigarette smokers often have several cigarettes a day. Obviously the more you smoke something the more likely it is that you will have cancer. Pot can result in brain damage, especially in young people. And as stated above, it can cause impairment, just as alcohol can. The reason pot is illegal and alcohol isn't? It's entirely a cultural thing. Similar to opium, consumed mainly by Asians, the criminalization of marijuana was used to repress the ethnic traditions of an ethnic minority.
Additionally, Mexicans were often interacting with black people in the south and giving them pot, which caused a great deal of paranoia as well. If you ask most white people what they think of a drunk or high white person they'll often tell you that the person was just out to have a good time, that they're partying, whatever.
However if you ask most white people what they think of a drunk non-white person, many will tell you that they're a user and abuser and a low life. I personally think this is wrong, white and non-white people are just as capable of using drugs and alcohol for recreational purposes on occasion as they are to abuse the substance. It is a very unfortunate stereotype. Other drugs were made illegal because of negative effects.
Heroin used to be perfectly legal and you used to be able to buy it and syringes in the Sears catalogue, however many people usually women and lower class people were using it and it was starting to have a negative effect.
Women and the lower class were becoming 'lazy' due to heroin use it pretty much knocks you out , thus making them incapable of producing goods lower class people were generally producers.
This loss of productivity led to a loss of profit for the rich business owners and who was in power when most of this was happening? Yep, rich business owners, who will make laws that help other rich business owners. Additionally, heroin use caused severe birth defects and infant mortality.