This is actually very poor logical thinking. Here's why.
Driving while intoxicated is illegal. If someone is engaging in illegal activity at a relatively common level they are unlikely to be disuaded from engaging in other illegal activity that is even less severe. The punishment for smoking marijuana is far less than that of driving while intoxicated. So its reasonable to say that MANY of those who do not smoke pot now because its illegal, but would if it was legalized, would NOT drive while intoxicated because they're people who actively try to stay within the law.
Furthermore, it assumes that everyone that will start smoking that didn't already would not already be engaging in a LEGAL mind influencing drink. IE, to believe that a LOT more people would DWI it would mean that there's a lot of people out there who don't DRINK, and don't smoke because its illegal, but would smoke if it was made legal.
Now, forgive me, but I don't believe that number is extremely large.
So you're going to have a small segment of people who will begin smoking, that didn't before, that didn't drink before either. Out of those you're going to need the ones that didn't avoid smoking due to fear of violation the law. That group then becomes your "most likely" people to DWI with the legalization of marijuana.
Frankly, I do not think that that number would be "A lot".