That data is very hard to come by, as we don't really have good numbers on how much marijuana gets used. Canada has very good data on how marijuana impaired accidents nearly quintupled when they legalized marijuana. The doctor that wrote that paper had this to say about comparative deadliness of the two substances:
“Stoned driving is as dangerous, if currently less deadly, than drunk driving. Many people don’t understand that, and have a false sense that driving while intoxicated on cannabis is safe, which is untrue.
“The odds of being involved in a motor vehicle crash when driving ‘stoned’ are approximately double those of sober driving, but significantly less than the 10 to 15 times increase when driving with a blood alcohol concentration of approximately 0.1,”
On it's face this would seem to agree with your assessment that marijuana is less deadly than alcohol. From a driving perspective it probably is. Although the paper went on to talk about how very few of the people who tested positive for marijuana only tested positive for marijuana. Most also tested positive for alcohol and that combination was significantly more deadly than alcohol alone.
It turns out that people that get stoned and drive tend to also be drunk, and the combination makes them even more dangerous than simply being drunk.
It also doesn't take into consideration increased risk factors long term for cardiovascular and other health related issues arising from marijuana use. Right now, admittedly the science behind that is murky. Numbers are hard to get for criminalized substances, and it is very difficult to separate the effects of marijuana from the effects of other drugs that often get used in combination with marijuana (alcohol being a prime example). That being the case smoking marijuana probably has most of the same health problems that smoking tobacco has. From the limited studies that have been done marijuana smokers can probably expect similar if not worse mortality rates. Not to mention the fact that while most marijuana users don't go on to use other "harder" drugs. There is a statistical link between marijuana use and later use of other drugs. Some of that makes sense. I have several friends that have told me that they have been given marijuana spiked with some other substance at one time or another.
It turns out that drug dealers are not nice people. Hanging out with them is a very serious health risk.
Just for the record, I am personally in favor of the decriminalization of marijuana, and I have even helped my niece get a medical marijuana card. I recognize that the science on some of this is still out. I just don't think that when the research is done, that marijuana is going to turn out to be as benign a substance as many marijuana advocates currently insist that it is. I think that it is at least safe to say that it is probably as harmful to your health as cigarette smoke, and long term cigarettes are invariably an early death sentence.