There are many parts to that question.
Way back when the 80 limit was introduced, there was very clear evidence that was very consistent across studies that when you looked at experimental studies that looked at impairment, there was very little doubt that people who were over a limit of 80 were impaired. You could demonstrate it on virtually any task you wanted to.
The other piece of that is the epidemiological literature which showed that at the point of 80, the risk of crash involvement increased exponentially. It was very clear, and that's been demonstrated repeatedly over the years. There is no question about the alcohol limit. We can play with the actual number a little bit, but there's no question about the evidence that supports it.
When it comes to other drugs, in particular, cannabis, it's a little more difficult. We can do studies where we give people cannabis and watch their performance, but the first thing we have to recognize is that the impairment due to cannabis doesn't look like the impairment due to alcohol. It can be very, very different. In particular, cannabis has a lot of cognitive impairments that are very difficult to see. In that context what we're looking at are things like decision-making, risk taking, executive functioning, memory, concentration. People who use cannabis will often say they concentrate better when they use cannabis, and maybe they think they do, but they're concentrating on one thing. Driving is a multi-modal task that requires you to concentrate on numerous different things at the same time. You need to pay attention, and they're unable to do that very well. They do not show the same kind of impairment as alcohol-impaired people do. At some point, you will see the kind of physical impairments, like the loss of balance and the inability to touch one's nose and fine motor discrimination, things like that, but we have to recognize that they're very different.
The other problem we have is that the epidemiological literature is a bit mixed when it comes to at what point cannabis increases the risk of crash involvement. In fact, we really don't have any information on that at all. What we have is a number of studies that look at the difference between non-cannabis-using drivers and cannabis-using drivers, so it's a positive or negative thing. Most of those studies, most but not all, show that there's increased risk associated with cannabis use while driving.
The situations are very different. There's a lot of work to do to help refine that scientific evidence over the next several years.